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.

src/Module.zig

@@ -2486,6 +2486,9 @@ pub const CompileError = error{
     /// In a comptime scope, a return instruction was encountered. This error is only seen when
     /// doing a comptime function call.
     ComptimeReturn,
+    /// In a comptime scope, a break instruction was encountered. This error is only seen when
+    /// evaluating a comptime block.
+    ComptimeBreak,
 };
 
 pub fn deinit(mod: *Module) void {
@@ -4446,6 +4449,7 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
             error.NeededSourceLocation => unreachable,
             error.GenericPoison => unreachable,
             error.ComptimeReturn => unreachable,
+            error.ComptimeBreak => unreachable,
             else => |e| return e,
         };
         if (opt_opv) |opv| {
@@ -4478,6 +4482,7 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
         error.NeededSourceLocation => @panic("zig compiler bug: NeededSourceLocation"),
         error.GenericPoison => @panic("zig compiler bug: GenericPoison"),
         error.ComptimeReturn => @panic("zig compiler bug: ComptimeReturn"),
+        error.ComptimeBreak => @panic("zig compiler bug: ComptimeBreak"),
         else => |e| return e,
     };