[AIX] Minor cleanup in AsmPrinter. [NFC]

- Extends the comments related to function descriptors, noting how they
are only used on AIX.

- Changes the condition used to gate the creation of the current function
symbol in AsmPrinter::SetupMachineFunction to reflect being AIX
specific. The creation of the symbol is different because of AIXs
linkage conventions, not because AIX uses function descriptors.

Differential Revision: https://reviews.llvm.org/D73115