GLES2: Use require_constant_initialization for g_Mutex

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/+/1832164
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

AUTHORS

@@ -58,3 +58,4 @@ Jérôme Duval
 Thomas Miller
 Till Rathmann
 Nick Shaforostov
+Jaime Bernardo

src/common/angleutils.h

@@ -345,4 +345,10 @@ std::string ToString(const T &value)
 #    define ANGLE_MAYBE_UNUSED
 #endif  // __has_cpp_attribute(maybe_unused)
 
+#if __has_cpp_attribute(require_constant_initialization)
+#    define ANGLE_REQUIRE_CONSTANT_INIT [[require_constant_initialization]]
+#else
+#    define ANGLE_REQUIRE_CONSTANT_INIT
+#endif  // __has_cpp_attribute(require_constant_initialization)
+
 #endif  // COMMON_ANGLEUTILS_H_

src/libGLESv2/global_state.cpp

@@ -35,9 +35,8 @@ namespace
 {
 static TLSIndex threadTLS = TLS_INVALID_INDEX;
 Debug *g_Debug            = nullptr;
-std::atomic<std::mutex *> g_Mutex;
-static_assert(std::is_trivially_constructible<decltype(g_Mutex)>::value,
-              "global mutex is not trivially constructible");
+
+ANGLE_REQUIRE_CONSTANT_INIT std::atomic<std::mutex *> g_Mutex(nullptr);
 static_assert(std::is_trivially_destructible<decltype(g_Mutex)>::value,
               "global mutex is not trivially destructible");