[clang] [Clang] Support libstdc++ workarounds when using `-E` (PR #210802)

[Corentin Jabot via cfe-commits]

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@@ -913,6 +913,48 @@ void Preprocessor::HandlePragmaHdrstop(Token &Tok) {
     SkippingUntilPragmaHdrStop = false;
 }
 
+bool Preprocessor::isPragmaSetPPStateMacro(IdentifierInfo *MacroName) {
+  return MacroName == Ident__GLIBCXX__;
+}
+
+void Preprocessor::HandlePragmaSetPPState(PragmaIntroducer Introducer,
+                                          Token &Tok) {
+  // Lex the macro name we want to set.
+  LexUnexpandedToken(Tok);
+  if (!Tok.getIdentifierInfo()) {
+    Diag(Tok.getLocation(), diag::err_pp_pragma_set_pp_state_expected_name);
+    return;
+  }
+
+  IdentifierInfo *MacroName = Tok.getIdentifierInfo();
+  if (!isPragmaSetPPStateMacro(MacroName)) {
+    Diag(Tok.getLocation(), diag::err_pp_pragma_set_pp_state_invalid_arg)
+        << MacroName;
+    return;
+  }
+
+  // Lex the integer argument.
+  Lex(Tok);
+  std::uint64_t Value;
+  if (!Tok.is(tok::numeric_constant) ||
+      !parseSimpleIntegerLiteral(Tok, Value)) {
+    Diag(Tok.getLocation(), diag::err_pp_pragma_set_pp_state_expected_int_after)
+        // Don't pass an IdentifierInfo* here to avoid quoting.
+        << MacroName->getName();
+    return;
+  }
+
+  // Update the state.
+  if (MacroName == Ident__GLIBCXX__) {
+    setStdLibCxxVersion(Value);
+  } else {
+    llvm_unreachable("forgot to handle a possible argument to __set_pp_state");
+  }
----------------
cor3ntin wrote:

And I think we are better off just looking at strings here - it's what we do elsewhere.
Comparing identifier info pointer makes sense where we do that very often - here we are likely to do it... a couple of times per TU at most

https://github.com/llvm/llvm-project/pull/210802