[PECOFF] Support response files.

If the driver finds a command line option in the form of "@filename", the
option will be replaced with the content of the given file. It's an error
if a response file cannot be read.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@203875 91177308-0d34-0410-b5e6-96231b3b80d8
Use short identifier, s/diagnostics/diag/.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@203876 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Add one more test for r203875.

This is to make sure that the Windows-style command line tokenizer is
working as expected.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@203877 91177308-0d34-0410-b5e6-96231b3b80d8