[LLD][COFF] Enable linking of __declspec(selectany) symbols from Clang and GCC

When annotating a symbol with __declspec(selectany), Clang assigns it
comdat 2 while GCC assigns it comdat 3. This patch enables two object
files that contain a __declspec(selectany) symbol, one created by gcc
and the other by clang, to be linked together instead of issuing a
duplicate symbol error.

Differential Revision: https://reviews.llvm.org/D73139

(cherry picked from commit 9dbc1ab2)