[Serialization] Use a more reliable macro to get the compiler tag

Use only the SWIFT_COMPILER_VERSION macro to check for swiftmodules
being written by the same compiler that reads it. In practice, it's the
macro used for release builds of the compiler.

rdar://96868333

lib/Basic/Version.cpp

@@ -507,5 +507,13 @@ bool isCurrentCompilerTagged() {
 #endif
 }
 
+StringRef getCurrentCompilerTag() {
+#ifdef SWIFT_COMPILER_VERSION
+  return SWIFT_COMPILER_VERSION;
+#else
+  return StringRef();
+#endif
+}
+
 } // end namespace version
 } // end namespace swift