[XCOFF] Implement parsing symbol table for xcoffobjfile and output as yaml format

Summary:
This patch implement parsing symbol table for xcoffobjfile and
output as yaml format. Parsing auxiliary entries of a symbol
will be in a separate patch.

The XCOFF object file (aix_xcoff.o) used in the test comes from
-bash-4.2$ cat test.c
extern int i;
extern int TestforXcoff;
int main()
{
i++;
TestforXcoff--;
}

Patch by DiggerLin

Reviewers: sfertile, hubert.reinterpretcast, MaskRay, daltenty

Differential Revision: https://reviews.llvm.org/D61532

llvm-svn: 361832