[PECOFF] Fix possible response file buffer overrun.
[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: f6089bf0e6284f307027cef4f64114ee9ebb0424@204420 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld