tools: avoid a string duplication

The auxiliary file records are contiguous and only contain the filename.
Construct a StringRef directly rather than copying to a temporary buffer.

Suggested by majnemer on IRC!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206139 91177308-0d34-0410-b5e6-96231b3b80d8
tools: remove duplication of coff_aux_file

Now that COFF::coff_aux_file is defined, use that rather than redefining the
type locally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206140 91177308-0d34-0410-b5e6-96231b3b80d8