[clang][AST] Fix generation with multiple external sources

ExternalASTSource::incrementGeneration returns the OldGeneration,
which must be taken into account in case it is delegated to the
topmost external source.

This obsoletes a long-standing downstream patch in Cling that was
previously submitted in https://reviews.llvm.org/D39714.

Author: hahnjo

interpreter/llvm-project/clang/lib/AST/ExternalASTSource.cpp

@@ -118,19 +118,20 @@ void ExternalASTSource::FindExternalLexicalDecls(
 void ExternalASTSource::getMemoryBufferSizes(MemoryBufferSizes &sizes) const {}
 
 uint32_t ExternalASTSource::incrementGeneration(ASTContext &C) {
-  uint32_t OldGeneration = CurrentGeneration;
-
   // Make sure the generation of the topmost external source for the context is
   // incremented. That might not be us.
   auto *P = C.getExternalSource();
-  if (P && P != this)
+  if (P && P != this) {
+    // The call itself returns the OldGeneration of the topmost external source.
     CurrentGeneration = P->incrementGeneration(C);
-  else {
-    // FIXME: Only bump the generation counter if the current generation number
-    // has been observed?
-    if (!++CurrentGeneration)
-      llvm::reportFatalUsageError("generation counter overflowed");
   }
 
+  uint32_t OldGeneration = CurrentGeneration;
+
+  // FIXME: Only bump the generation counter if the current generation number
+  // has been observed?
+  if (!++CurrentGeneration)
+    llvm::reportFatalUsageError("generation counter overflowed");
+
   return OldGeneration;
 }