Add RawIdentifiers as a language feature.

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

Author: allevato

include/swift/Basic/Features.def

@@ -215,6 +215,7 @@ LANGUAGE_FEATURE(ObjCImplementation, 436, "@objc @implementation extensions")
 LANGUAGE_FEATURE(NonescapableTypes, 446, "Nonescapable types")
 LANGUAGE_FEATURE(BuiltinEmplaceTypedThrows, 0, "Builtin.emplace typed throws")
 SUPPRESSIBLE_LANGUAGE_FEATURE(MemorySafetyAttributes, 458, "@unsafe attribute")
+LANGUAGE_FEATURE(RawIdentifiers, 451, "Raw identifiers")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

lib/AST/FeatureSet.cpp

@@ -330,6 +330,7 @@ UNINTERESTING_FEATURE(DebugDescriptionMacro)
 UNINTERESTING_FEATURE(ReinitializeConsumeInMultiBlockDefer)
 UNINTERESTING_FEATURE(SE427NoInferenceOnExtension)
 UNINTERESTING_FEATURE(TrailingComma)
+UNINTERESTING_FEATURE(RawIdentifiers)
 
 static ABIAttr *getABIAttr(Decl *decl) {
   if (auto pbd = dyn_cast<PatternBindingDecl>(decl))