Implement ANGLE_FALLTHROUGH macro.

Use it instead of ABSL_FALLTHROUGH_INTENDED.
(ANGLE shouldn't use absl unless angle_enable_absl is true.
This is breaking the roll of ANGLE into Dawn.)

Bug: angleproject:6647
Change-Id: I47f9a00841c4605fca52b17eeb68c0b1da5004c0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3258008
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>

src/common/angleutils.h

@@ -430,4 +430,10 @@ inline bool IsLittleEndian()
 #    define ANGLE_REQUIRE_CONSTANT_INIT
 #endif  // __has_cpp_attribute(require_constant_initialization)
 
+#if __has_cpp_attribute(clang::fallthrough)
+#    define ANGLE_FALLTHROUGH [[clang::fallthrough]]
+#else
+#    define ANGLE_FALLTHROUGH
+#endif
+
 #endif  // COMMON_ANGLEUTILS_H_

src/compiler/translator/TranslatorMetalDirect/EmitMetal.cpp

@@ -779,7 +779,7 @@ void GenMetalTraverser::emitPostQualifier(const EmitVariableDeclarationConfig &e
     {
         case TQualifier::EvqPosition:
             isInvariant = decl.type().isInvariant();
-            ABSL_FALLTHROUGH_INTENDED;
+            ANGLE_FALLTHROUGH;
         case TQualifier::EvqFragCoord:
             mOut << " [[position]]";
             break;