Sema: fix comptime break semantics

Previously, breaking from an outer block at comptime would result in
incorrect control flow. Now there is a mechanism, `error.ComptimeBreak`,
similar to `error.ComptimeReturn`, to send comptime control flow further
up the stack, to its matching block.

This commit also introduces a new log scope. To use it, pass
`--debug-log sema_zir` and you will see 1 line per ZIR instruction
semantically analyzed. This is useful when you want to understand what
comptime control flow is doing while debugging the compiler.

One more `switch` test case is passing.

1 files changed, 21 insertions, 0 deletions

diff --git a/test/behavior/switch.zig b/test/behavior/switch.zig
index caec804ec2..c435d70b9f 100644
--- a/test/behavior/switch.zig
+++ b/test/behavior/switch.zig
@@ -326,3 +326,24 @@ test "anon enum literal used in switch on union enum" {
         },
     }
 }
+
+test "switch all prongs unreachable" {
+    try testAllProngsUnreachable();
+    comptime try testAllProngsUnreachable();
+}
+
+fn testAllProngsUnreachable() !void {
+    try expect(switchWithUnreachable(1) == 2);
+    try expect(switchWithUnreachable(2) == 10);
+}
+
+fn switchWithUnreachable(x: i32) i32 {
+    while (true) {
+        switch (x) {
+            1 => return 2,
+            2 => break,
+            else => continue,
+        }
+    }
+    return 10;
+}