[ASTGen] Move getIdentifier logic to ASTGen side

Ideally ASTBriding should only do the neccessary work for briding. All
non-trivial logic should be in ASTGen.

Author: rintaro

lib/AST/ASTBridging.cpp

@@ -110,16 +110,7 @@ BridgedDeclNameLoc_createParsed(BridgedSourceLoc cBaseNameLoc) {
 
 BridgedIdentifier BridgedASTContext_getIdentifier(BridgedASTContext cContext,
                                                   BridgedStringRef cStr) {
-  StringRef str = cStr.unbridged();
-  if (str.size() == 1 && str.front() == '_')
-    return BridgedIdentifier();
-
-  // If this was a back-ticked identifier, drop the back-ticks.
-  if (str.size() >= 2 && str.front() == '`' && str.back() == '`') {
-    str = str.drop_front().drop_back();
-  }
-
-  return cContext.unbridged().getIdentifier(str);
+  return cContext.unbridged().getIdentifier(cStr.unbridged());
 }
 
 bool BridgedASTContext_langOptsHasFeature(BridgedASTContext cContext,

lib/ASTGen/Sources/ASTGen/Bridge.swift

@@ -167,10 +167,14 @@ extension TokenSyntax {
   /// - Parameter astgen: The visitor providing the `ASTContext`.
   @inline(__always)
   func bridgedIdentifier(in astgen: ASTGenVisitor) -> BridgedIdentifier {
-    var text = self.text
-    return text.withBridgedString { bridged in
-      astgen.ctx.getIdentifier(bridged)
+    var text = self.rawText
+    if rawText == "_" {
+      return nil
+    }
+    if rawText.count > 2 && rawText.hasPrefix("`") && rawText.hasSuffix("`") {
+      text = .init(rebasing: text.dropFirst().dropLast())
     }
+    return astgen.ctx.getIdentifier(rawText.bridged)
   }
 
   /// Obtains a bridged, `ASTContext`-owned copy of this token's text, and its bridged start location in the