[clang][Preprocessor] Add peekNextPPToken, makes look ahead next token without side-effects (#143898)

This PR introduce a new function `peekNextPPToken`. It's an extension of
`isNextPPTokenLParen` and can makes look ahead one token in preprocessor
without side-effects.

It's also the 1st part of
https://github.com/llvm/llvm-project/pull/107168 and it was used to look
ahead next token then determine whether current lexing pp directive is
one of pp-import or pp-module directive.

At the start of phase 4 an import or module token is treated as starting
a directive and are converted to their respective keywords iff:

 - After skipping horizontal whitespace are
    - at the start of a logical line, or
    - preceded by an export at the start of the logical line.
- Are followed by an identifier pp token (before macro expansion), or
    - <, ", or : (but not ::) pp tokens for import, or
    - ; for module
Otherwise the token is treated as an identifier.

---------

Signed-off-by: yronglin <yronglin777@gmail.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/clang/lib/Lex/TokenLexer.cpp b/clang/lib/Lex/TokenLexer.cpp
index 6e93416..fbb8c42 100644
--- a/clang/lib/Lex/TokenLexer.cpp
+++ b/clang/lib/Lex/TokenLexer.cpp
@@ -921,13 +921,13 @@ bool TokenLexer::pasteTokens(Token &LHSTok, ArrayRef<Token> TokenStream,
 }
 
 /// isNextTokenLParen - If the next token lexed will pop this macro off the
-/// expansion stack, return 2.  If the next unexpanded token is a '(', return
-/// 1, otherwise return 0.
-unsigned TokenLexer::isNextTokenLParen() const {
+/// expansion stack, return std::nullopt, otherwise return the next unexpanded
+/// token.
+std::optional<Token> TokenLexer::peekNextPPToken() const {
   // Out of tokens?
   if (isAtEnd())
-    return 2;
-  return Tokens[CurTokenIdx].is(tok::l_paren);
+    return std::nullopt;
+  return Tokens[CurTokenIdx];
 }
 
 /// isParsingPreprocessorDirective - Return true if we are in the middle of a