[Clang][Lex][NFC] Assert getExternalSource() in updateOutOfDateIdentifier (#140137)

Static analysis flagged the unconditional access of getExternalSource().
We don't initialize ExternalSource during construction but via
setExternalSource(). If this is not set it will violate the invariant
covered by the assert.

Author: shafik

clang/lib/Lex/Preprocessor.cpp

@@ -759,6 +759,8 @@ void Preprocessor::HandlePoisonedIdentifier(Token & Identifier) {
 
 void Preprocessor::updateOutOfDateIdentifier(const IdentifierInfo &II) const {
   assert(II.isOutOfDate() && "not out of date");
+  assert(getExternalSource() &&
+         "getExternalSource() should not return nullptr");
   getExternalSource()->updateOutOfDateIdentifier(II);
 }