[XCOFF][AIX] Use unique section names for LSDA and EH info sections with -ffunction-sections

Summary:
When -ffunction-sections is on, this patch makes the compiler to generate unique LSDA and EH info sections for functions on AIX by appending the function name to the section name as a suffix. This will allow the AIX linker to garbage-collect unused function.

Reviewed by: MaskRay, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D124855