[XCOFF] Don't emit non-external labels in the symbol table and handle MCSA_LGlobal

Summary:
We need to handle the  MCSA_LGlobal case in emitSymbolAttribute for functions marked internal in the IR so that the
appropriate storage class is emitted on the function descriptor csect.  As part of this we need to make sure that external
labels are not emitted into the symbol table, so we don't emit the descriptor label in the object writing path.

Reviewers: jasonliu, DiggerLin, hubert.reinterpretcast

Reviewed By: jasonliu

Subscribers: Xiangling_L, wuzish, nemanjai, hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74968