This is a fix for PR43315. An assertion error is hit for this minimal example:
//clang -cc1 -triple x86_64-- -S tstVMStructRC-min.cpp #define a __attribute__((__cdecl__, __regparm__(0))) int (a b)(); // Assertion `Chunk.Kind == DeclaratorChunk::Function' failed.
This is because we do not cover the case in the FunctionTypeUnwrapper where it receives a MacroQualifiedType. We have not run into this earlier because this is a unique case where the __attribute__ contains both __cdecl__ and __regparm__ (in that order), and we are compiling for x86_64. Changing the architecture or the order of __cdecl__ and __regparm__ does not raise the assertion.