test: update for CallingConvention changes

This also includes some compiler and std changes to correct error
messages which weren't properly updated before.

test/cases/compile_errors/invalid_extern_function_call.zig

@@ -1,4 +1,4 @@
-const x = @extern(*const fn () callconv(.C) void, .{ .name = "foo" });
+const x = @extern(*const fn () callconv(.c) void, .{ .name = "foo" });
 
 export fn entry0() void {
     comptime x();