[clang] [Modules] Fix an identifier hiding a function-like macro definition. (PR #135471)

[Jan Svoboda via cfe-commits]

================
@@ -3871,7 +3874,8 @@ class ASTIdentifierTableTrait {
     if (isInterestingIdentifier(II, MacroOffset)) {
       DataLen += 2; // 2 bytes for builtin ID
       DataLen += 2; // 2 bytes for flags
-      if (MacroOffset)
+      if (MacroOffset || (II->hasMacroDefinition() &&
+                          II->hasFETokenInfoChangedSinceDeserialization()))
----------------
jansvoboda11 wrote:

Ah, I see that `ASTReader` does this for each `ModuleFile`:
```c++
    for (auto Offset : F.PreloadIdentifierOffsets) {
      // ...
      IdentifierInfo *II;
      if (!PP.getLangOpts().CPlusPlus) {
        // Identifiers present in both the module file and the importing
        // instance are marked out-of-date so that they can be deserialized
        // on next use via ASTReader::updateOutOfDateIdentifier().
        // Identifiers present in the module file but not in the importing
        // instance are ignored for now, preventing growth of the identifier
        // table. They will be deserialized on first use via ASTReader::get().
        auto It = PP.getIdentifierTable().find(Key);
        if (It == PP.getIdentifierTable().end())
          continue;
        II = It->second;
      } else {
        // With C++ modules, not many identifiers are considered interesting.
        // All identifiers in the module file can be placed into the identifier
        // table of the importing instance and marked as out-of-date. This makes
        // ASTReader::get() a no-op, and deserialization will take place on
        // first/next use via ASTReader::updateOutOfDateIdentifier().
        II = &PP.getIdentifierTable().getOwn(Key);
      }
      // ...
      markIdentifierFromAST(*this, *II, /*IsModule=*/true);
      //...
    }
```
and "__Q" already is in the table, so that works out. Makes sense, thanks for walking me through it.

https://github.com/llvm/llvm-project/pull/135471