[COFF] Don't try to add quotes to already quoted linker directives

If a linker directive is already quoted, don't try to quote it again, otherwise it creates a mess.
This pops up in places like:
#pragma comment(linker,"\"/foo bar'\"")

Differential Revision: http://reviews.llvm.org/D6792

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224998 91177308-0d34-0410-b5e6-96231b3b80d8