Zcu: store LazySrcLoc in error messages

This change modifies `Zcu.ErrorMsg` to store a `Zcu.LazySrcLoc` rather
than a `Zcu.SrcLoc`. Everything else is dominoes.

The reason for this change is incremental compilation. If a failed
`AnalUnit` is up-to-date on an update, we want to re-use the old error
messages. However, the file containing the error location may have been
modified, and `SrcLoc` cannot survive such a modification. `LazySrcLoc`
is designed to be correct across incremental updates. Therefore, we
defer source location resolution until `Compilation` gathers the compile
errors into the `ErrorBundle`.

src/codegen/llvm.zig

@@ -4644,7 +4644,7 @@ pub const DeclGen = struct {
         const o = dg.object;
         const gpa = o.gpa;
         const mod = o.module;
-        const src_loc = dg.decl.navSrcLoc(mod).upgrade(mod);
+        const src_loc = dg.decl.navSrcLoc(mod);
         dg.err_msg = try Module.ErrorMsg.create(gpa, src_loc, "TODO (LLVM): " ++ format, args);
         return error.CodegenFail;
     }