[index] Factor out the dependency between the core indexing walker and SourceKit

The goal is to be able to move the core IndexSwiftASTWalker code out of
SourceKit, leaving only the serialization bits behind.

Mostly this replaces some direct uses of UIdent strings with explicit
enums, and then adds the translation code to produce those enums and to
convert them into UIdents in SourceKit.

rdar://problem/22348041

Author: benlangmuir

tools/SourceKit/include/SourceKit/Core/LangSupport.h

@@ -38,9 +38,9 @@ struct EntityInfo {
   TypeKind EntityType = Base;
 
   UIdent Kind;
-  llvm::SmallString<32> Name;
-  llvm::SmallString<64> USR;
-  llvm::SmallString<16> Group;
+  StringRef Name;
+  StringRef USR;
+  StringRef Group;
   unsigned Line = 0;
   unsigned Column = 0;
 
@@ -57,7 +57,7 @@ struct FuncDeclEntityInfo : public EntityInfo {
 };
 
 struct CallRefEntityInfo : public EntityInfo {
-  llvm::SmallString<64> ReceiverUSR;
+  StringRef ReceiverUSR;
   bool IsDynamic = false;
 
   CallRefEntityInfo() : EntityInfo(CallReference) { }