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/c.zig

@@ -637,7 +637,7 @@ pub const DeclGen = struct {
         const zcu = dg.zcu;
         const decl_index = dg.pass.decl;
         const decl = zcu.declPtr(decl_index);
-        const src_loc = decl.navSrcLoc(zcu).upgrade(zcu);
+        const src_loc = decl.navSrcLoc(zcu);
         dg.error_msg = try Zcu.ErrorMsg.create(dg.gpa, src_loc, format, args);
         return error.AnalysisFail;
     }