[hwasan] Move __hwasan_thread_enter/__hwasan_thread_exit out of namespace (#72123)

Due to a GCC bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25940),
GCC doesn't consider extern "C" functions with the same name but
different namespace to be the same. As such, the default visibility
attribute (on a declaration outside the namespace) doesn't get applied
to the definition in the namespace and the symbol is not exported.

This came up as an ABI diff when switching between gcc and clang for
compiling compiler-rt.

Author: nikic

compiler-rt/lib/hwasan/hwasan_linux.cpp

@@ -294,25 +294,6 @@ void InstallAtExitHandler() { atexit(HwasanAtExit); }
 
 // ---------------------- TSD ---------------- {{{1
 
-extern "C" void __hwasan_thread_enter() {
-  hwasanThreadList().CreateCurrentThread()->EnsureRandomStateInited();
-}
-
-extern "C" void __hwasan_thread_exit() {
-  Thread *t = GetCurrentThread();
-  // Make sure that signal handler can not see a stale current thread pointer.
-  atomic_signal_fence(memory_order_seq_cst);
-  if (t) {
-    // Block async signals on the thread as the handler can be instrumented.
-    // After this point instrumented code can't access essential data from TLS
-    // and will crash.
-    // Bionic already calls __hwasan_thread_exit with blocked signals.
-    if (SANITIZER_GLIBC)
-      BlockSignals();
-    hwasanThreadList().ReleaseThread(t);
-  }
-}
-
 #  if HWASAN_WITH_INTERCEPTORS
 static pthread_key_t tsd_key;
 static bool tsd_key_inited = false;
@@ -561,4 +542,25 @@ void InstallAtExitCheckLeaks() {
 
 }  // namespace __hwasan
 
+using namespace __hwasan;
+
+extern "C" void __hwasan_thread_enter() {
+  hwasanThreadList().CreateCurrentThread()->EnsureRandomStateInited();
+}
+
+extern "C" void __hwasan_thread_exit() {
+  Thread *t = GetCurrentThread();
+  // Make sure that signal handler can not see a stale current thread pointer.
+  atomic_signal_fence(memory_order_seq_cst);
+  if (t) {
+    // Block async signals on the thread as the handler can be instrumented.
+    // After this point instrumented code can't access essential data from TLS
+    // and will crash.
+    // Bionic already calls __hwasan_thread_exit with blocked signals.
+    if (SANITIZER_GLIBC)
+      BlockSignals();
+    hwasanThreadList().ReleaseThread(t);
+  }
+}
+
 #endif  // SANITIZER_FREEBSD || SANITIZER_LINUX || SANITIZER_NETBSD