Object/COFF: Define coff_symbol_generic.

Object/COFF: Define coff_symbol_generic.

If you only need Name and Value fields in the COFF symbol,
you don't need to distinguish 32 bit and 64 bit COFF symbols.
These fields start at the same offsets and have the same size.

This data strucutre is one pointer smaller than COFFSymbolRef
thus slightly efficient. I'll use this class in LLD as we create
millions of LLD symbol objects that currently contain COFFSymbolRef.
Shaving off 8 byte (or 4 byte on 32 bit) from that class actually
matters becasue of the number of objects we create in LLD.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241024 91177308-0d34-0410-b5e6-96231b3b80d8