[clangd] Get rid of lexer usage in AST.cpp

clang-tools-extra/clangd/AST.h

 NestedNameSpecifierLoc getQualifierLoc(const NamedDecl &ND);
 
 // Returns a type corresponding to a declaration of that type.
 // Unlike the method on ASTContext, attempts to preserve the type as-written
 // (i.e. vector<T*> rather than vector<type-parameter-0-0 *>.
 QualType declaredType(const TypeDecl *D);
 
 /// Retrieves the deduced type at a given location (auto, decltype).
-/// Retuns None unless Loc starts an auto/decltype token.
 /// It will return the underlying type.
 llvm::Optional<QualType> getDeducedType(ASTContext &, SourceLocation Loc);
 
 /// Gets the nested name specifier necessary for spelling \p ND in \p
 /// DestContext, at \p InsertionPoint. It selects the shortest suffix of \p ND
 /// such that it is visible in \p DestContext.
 /// Returns an empty string if no qualification is necessary. For example, if
 /// you want to qualify clang::clangd::bar::foo in clang::clangd::x, this

clang-tools-extra/clangd/AST.cpp

 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/PrettyPrinter.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Index/USRGeneration.h"
-#include "clang/Lex/Lexer.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/raw_ostream.h"
 #include <string>
public:
   }
 
   QualType DeducedType;
 };
 } // namespace
 
 llvm::Optional<QualType> getDeducedType(ASTContext &ASTCtx,
                                         SourceLocation Loc) {
-  Token Tok;
-  // Only try to find a deduced type if the token is auto or decltype.
-  if (!Loc.isValid() ||
-      Lexer::getRawToken(Loc, Tok, ASTCtx.getSourceManager(),
-                         ASTCtx.getLangOpts(), false) ||
-      !Tok.is(tok::raw_identifier) ||
-      !(Tok.getRawIdentifier() == "auto" ||
-        Tok.getRawIdentifier() == "decltype")) {
+  if (!Loc.isValid())
     return {};
-  }
   DeducedTypeVisitor V(Loc);
   V.TraverseAST(ASTCtx);
   if (V.DeducedType.isNull())
     return llvm::None;
   return V.DeducedType;
 }
 
 std::string getQualification(ASTContext &Context,