A number of backends (AArch64, MIPS, ARM) have been using MCContext::reportError to report issues such as out-of-range fixup values in their TgtAsmBackend. This is great, but because MCContext couldn't easily be threaded through to the adjustFixupValue helper function from its usual callsite (applyFixup), these backends ended up adding an MCContext* argument and adding another call to applyFixup to processFixupValue. If we just add an MCContext parameter to applyFixup, this is unnecessary, and even better - applyFixup can take a reference to MCContext rather than a potentially null pointer.
See D18402 for some relevant discussion.