compiler: rework backend pipeline to separate codegen and link

The idea here is that instead of the linker calling into codegen,
instead codegen should run before we touch the linker, and after MIR is
produced, it is sent to the linker. Aside from simplifying the call
graph (by preventing N linkers from each calling into M codegen
backends!), this has the huge benefit that it is possible to
parallellize codegen separately from linking. The threading model can
look like this:

* 1 semantic analysis thread, which generates AIR
* N codegen threads, which process AIR into MIR
* 1 linker thread, which emits MIR to the binary

The codegen threads are also responsible for `Air.Legalize` and
`Air.Liveness`; it's more efficient to do this work here instead of
blocking the main thread for this trivially parallel task.

I have repurposed the `Zcu.Feature.separate_thread` backend feature to
indicate support for this 1:N:1 threading pattern. This commit makes the
C backend support this feature, since it was relatively easy to divorce
from `link.C`: it just required eliminating some shared buffers. Other
backends don't currently support this feature. In fact, they don't even
compile -- the next few commits will fix them back up.

7 files changed, 8 insertions, 8 deletions

diff --git a/src/libs/freebsd.zig b/src/libs/freebsd.zig
index 47fef32773..98d4a42f91 100644
--- a/src/libs/freebsd.zig
+++ b/src/libs/freebsd.zig
@@ -1004,7 +1004,7 @@ fn queueSharedObjects(comp: *Compilation, so_files: BuiltSharedObjects) void {
         }
     }
 
-    comp.queueLinkTasks(task_buffer[0..task_buffer_i]);
+    comp.queuePrelinkTasks(task_buffer[0..task_buffer_i]);
 }
 
 fn buildSharedLib(
diff --git a/src/libs/glibc.zig b/src/libs/glibc.zig
index cc781c5472..c1146d933d 100644
--- a/src/libs/glibc.zig
+++ b/src/libs/glibc.zig
@@ -1170,7 +1170,7 @@ fn queueSharedObjects(comp: *Compilation, so_files: BuiltSharedObjects) void {
         }
     }
 
-    comp.queueLinkTasks(task_buffer[0..task_buffer_i]);
+    comp.queuePrelinkTasks(task_buffer[0..task_buffer_i]);
 }
 
 fn buildSharedLib(
diff --git a/src/libs/libcxx.zig b/src/libs/libcxx.zig
index 17a7d3d29e..eb9f5df855 100644
--- a/src/libs/libcxx.zig
+++ b/src/libs/libcxx.zig
@@ -308,7 +308,7 @@ pub fn buildLibCxx(comp: *Compilation, prog_node: std.Progress.Node) BuildError!
     assert(comp.libcxx_static_lib == null);
     const crt_file = try sub_compilation.toCrtFile();
     comp.libcxx_static_lib = crt_file;
-    comp.queueLinkTaskMode(crt_file.full_object_path, &config);
+    comp.queuePrelinkTaskMode(crt_file.full_object_path, &config);
 }
 
 pub fn buildLibCxxAbi(comp: *Compilation, prog_node: std.Progress.Node) BuildError!void {
@@ -504,7 +504,7 @@ pub fn buildLibCxxAbi(comp: *Compilation, prog_node: std.Progress.Node) BuildErr
     assert(comp.libcxxabi_static_lib == null);
     const crt_file = try sub_compilation.toCrtFile();
     comp.libcxxabi_static_lib = crt_file;
-    comp.queueLinkTaskMode(crt_file.full_object_path, &config);
+    comp.queuePrelinkTaskMode(crt_file.full_object_path, &config);
 }
 
 pub fn addCxxArgs(
diff --git a/src/libs/libtsan.zig b/src/libs/libtsan.zig
index 8a5ffd2eab..0c59d85bc5 100644
--- a/src/libs/libtsan.zig
+++ b/src/libs/libtsan.zig
@@ -325,7 +325,7 @@ pub fn buildTsan(comp: *Compilation, prog_node: std.Progress.Node) BuildError!vo
     };
 
     const crt_file = try sub_compilation.toCrtFile();
-    comp.queueLinkTaskMode(crt_file.full_object_path, &config);
+    comp.queuePrelinkTaskMode(crt_file.full_object_path, &config);
     assert(comp.tsan_lib == null);
     comp.tsan_lib = crt_file;
 }
diff --git a/src/libs/libunwind.zig b/src/libs/libunwind.zig
index 945689ebab..ccea649c17 100644
--- a/src/libs/libunwind.zig
+++ b/src/libs/libunwind.zig
@@ -195,7 +195,7 @@ pub fn buildStaticLib(comp: *Compilation, prog_node: std.Progress.Node) BuildErr
     };
 
     const crt_file = try sub_compilation.toCrtFile();
-    comp.queueLinkTaskMode(crt_file.full_object_path, &config);
+    comp.queuePrelinkTaskMode(crt_file.full_object_path, &config);
     assert(comp.libunwind_static_lib == null);
     comp.libunwind_static_lib = crt_file;
 }
diff --git a/src/libs/musl.zig b/src/libs/musl.zig
index d208b09827..21aeee98b5 100644
--- a/src/libs/musl.zig
+++ b/src/libs/musl.zig
@@ -278,7 +278,7 @@ pub fn buildCrtFile(comp: *Compilation, in_crt_file: CrtFile, prog_node: std.Pro
             errdefer comp.gpa.free(basename);
 
             const crt_file = try sub_compilation.toCrtFile();
-            comp.queueLinkTaskMode(crt_file.full_object_path, &config);
+            comp.queuePrelinkTaskMode(crt_file.full_object_path, &config);
             {
                 comp.mutex.lock();
                 defer comp.mutex.unlock();
diff --git a/src/libs/netbsd.zig b/src/libs/netbsd.zig
index 718861bf5c..aab75cce49 100644
--- a/src/libs/netbsd.zig
+++ b/src/libs/netbsd.zig
@@ -669,7 +669,7 @@ fn queueSharedObjects(comp: *Compilation, so_files: BuiltSharedObjects) void {
         }
     }
 
-    comp.queueLinkTasks(task_buffer[0..task_buffer_i]);
+    comp.queuePrelinkTasks(task_buffer[0..task_buffer_i]);
 }
 
 fn buildSharedLib(