[UniqueLinkageName] Use exsiting GlobalDecl object instead of reconstructing one.

C++ constructors/destructors need to go through a different constructor to construct a GlobalDecl object in order to retrieve their linkage type. This causes an assert failure in the default constructor of GlobalDecl. I'm chaning it to using the exsiting GlobalDecl object.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D102356