A static assert to verify that the global mutex g_Mutex is trivially
constructed fails to compile with clang when using the STL shipped
with Visual Studio 2019.

Use __attribute__((require_constant_initialization)) instead to verify
for constant initialization.

BUG=angleproject:3936

Change-Id: I5969762ad5a99033143513d7c4992344da276b1a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1832164Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>