C backend: Improve lowering of Zig types to C types

1. Changed Zig pointers to functions to be typedef'd so then we can
   treat them the same as other types.

2. Distinguished between const slices (zig_L prefix) and mut slices
   (zig_M prefix).

3. Changed lowering of Zig "const pointers" (e.g. *const u8) to to C
   "pointers to const" (e.g. const char *) rather than C "const
   pointers" (e.g.  char * const)

4. Ensured that all typedefs are "linked" even if the decl doesn't
   require any forward declarations

5. Added test that exercises function pointer type rendering

6. Changed .slice_ptr instruction to allocate pointer local rather than
   a uintptr_t local

1 files changed, 4 insertions, 1 deletions

diff --git a/src/link/C.zig b/src/link/C.zig
index 611eadb045..5ca389ae62 100644
--- a/src/link/C.zig
+++ b/src/link/C.zig
@@ -362,7 +362,7 @@ fn flushDecl(self: *C, f: *Flush, decl: *const Module.Decl) FlushDeclError!void
     const decl_block = self.decl_table.getPtr(decl).?;
     const gpa = self.base.allocator;
 
-    if (decl_block.fwd_decl.items.len != 0) {
+    if (decl_block.typedefs.count() != 0) {
         try f.typedefs.ensureUnusedCapacity(gpa, @intCast(u32, decl_block.typedefs.count()));
         var it = decl_block.typedefs.iterator();
         while (it.next()) |new| {
@@ -371,6 +371,9 @@ fn flushDecl(self: *C, f: *Flush, decl: *const Module.Decl) FlushDeclError!void
                 try f.err_typedef_buf.appendSlice(gpa, new.value_ptr.rendered);
             }
         }
+    }
+
+    if (decl_block.fwd_decl.items.len != 0) {
         const buf = decl_block.fwd_decl.items;
         try f.all_buffers.append(gpa, .{
             .iov_base = buf.ptr,