Fix a typo in head comment of CurPPLexer.

In head comment of CurPPLexer field of class Preprocessor,
'The current top of the stack what we're lexing from' should be
'The current top of the stack that we're lexing from'.

Differential Revision: https://reviews.llvm.org/D149709

Author: zhouzhouyi-hub

clang/include/clang/Lex/Preprocessor.h

@@ -729,7 +729,7 @@ class Preprocessor {
   /// Only one of CurLexer, or CurTokenLexer will be non-null.
   std::unique_ptr<Lexer> CurLexer;
 
-  /// The current top of the stack what we're lexing from
+  /// The current top of the stack that we're lexing from
   /// if not expanding a macro.
   ///
   /// This is an alias for CurLexer.