Remove -Wno-thread-safety-negative

-Wthread-safety-negative was disabled because it requires locks to
support an operator!(), and std::mutex in libc++ does not.  Disabling
it everywhere was preventing it being used for modules that want to
opt in to using it and provide their own lock that supports
operator!().  Modules that use -Weverything should either stop using
 -Weverything (preferred), or add -Wno-thread-safety-negative.

Bug: 143713277
Test: m native
Change-Id: I0b84679a806a21b16cabe7f739fde61c3efe2cbe

cc/config/clang.go

@@ -139,9 +139,7 @@ func init() {
 
 	pctx.StaticVariable("ClangExtraCppflags", strings.Join([]string{
 		// Enable clang's thread-safety annotations in libcxx.
-		// Turn off -Wthread-safety-negative, to avoid breaking projects that use -Weverything.
 		"-D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS",
-		"-Wno-thread-safety-negative",
 
 		// libc++'s math.h has an #include_next outside of system_headers.
 		"-Wno-gnu-include-next",