[AIX] Implement function descriptor on SDAG

Summary:
(1) Function descriptor on AIX
On AIX, a called routine may have 2 distinct symbols associated with it:
 * A function descriptor (Name)
 * A function entry point (.Name)

The descriptor structure on AIX is the same as those in the ELF V1 ABI:
 * The address of the entry point of the function.
 * The TOC base address for the function.
 * The environment pointer.

The descriptor symbol uses the same name as the source level function in C.
The function entry point is analogous to the symbol we would generate for a
 function in a non-descriptor-based ABI, except that it is renamed by
prepending a ".".

Which symbol gets referenced depends on the context:
 * Taking the address of the function references the descriptor symbol.
 * Calling the function references the entry point symbol.

(2) Speaking of implementation on AIX, for direct function call target, we
 create proper MCSymbol SDNode(e.g . ".foo") while constructing SDAG to
 replace original TargetGlobalAddress SDNode. Then down the path, we can
 take advantage of this MCSymbol.

Patch by: Xiangling_L

Reviewed by: sfertile, hubert.reinterpretcast, jasonliu, syzaara

Differential Revision: https://reviews.llvm.org/D62532

llvm-svn: 362735