doxygen parsing buglets

If you look at the doxygen page for MCAsmBackend it only lists RISCV, Mips, ARM and AVR as implementers of applyFixup(). I think this is because the parser sees RISCVAsmBackend::applyFixup in ./RISCV/MCTargetDesc/RISCVAsmBackend.cpp or its .h file but then doesn’t recognize/match the implementation of applyFixup() in ./AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

Also, it says that applyFixup() is referenced by evaluateTargetFixup(). The best I can tell from grepping the backend sources is that evaluateTargetFixup() was added by/for the RISCV backend and is only used by that backend. But looking at the source, RISCVAsmBackend::evaluateTargetFixup doesn’t in fact call applyFixup().