[PECOFF] Fix possible response file buffer overrun.

Response file is not NUL terminated, so when creating a StringRef for
the buffer contents, we need to pass the buffer size as well as the
pointer pointing to the buffer.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204420 91177308-0d34-0410-b5e6-96231b3b80d8