[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

include/swift/Basic/Version.h

@@ -191,9 +191,12 @@ StringRef getSwiftRevision();
 
 /// Is the running compiler built with a version tag for distribution?
 /// When true, \c Version::getCurrentCompilerVersion returns a valid version
-/// and \c getSwiftRevision returns the version tuple in string format.
+/// and \c getCurrentCompilerTag returns the version tuple in string format.
 bool isCurrentCompilerTagged();
 
+/// Retrieves the distribtion tag of the running compiler, if any.
+StringRef getCurrentCompilerTag();
+
 } // end namespace version
 } // end namespace swift