Add RawIdentifiers as a language feature.

This lets clients test `#if hasFeature(RawIdentifiers)` to
determine compiler support.

Author: allevato

include/swift/Basic/Features.def

@@ -204,6 +204,7 @@ LANGUAGE_FEATURE(IsolatedAny2, 431, "@isolated(any) function types")
 LANGUAGE_FEATURE(ObjCImplementation, 436, "@objc @implementation extensions")
 LANGUAGE_FEATURE(NonescapableTypes, 446, "Nonescapable types")
 LANGUAGE_FEATURE(BuiltinEmplaceTypedThrows, 0, "Builtin.emplace typed throws")
+LANGUAGE_FEATURE(RawIdentifiers, 451, "Raw identifiers")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

lib/AST/FeatureSet.cpp

@@ -328,6 +328,7 @@ UNINTERESTING_FEATURE(DebugDescriptionMacro)
 UNINTERESTING_FEATURE(ReinitializeConsumeInMultiBlockDefer)
 UNINTERESTING_FEATURE(SE427NoInferenceOnExtension)
 UNINTERESTING_FEATURE(TrailingComma)
+UNINTERESTING_FEATURE(RawIdentifiers)
 
 static bool usesFeatureAllowUnsafeAttribute(Decl *decl) {
   return decl->getAttrs().hasAttribute<UnsafeAttr>();