[PECOFF] Parse /manifest command line option.

The manifest file is an XML file that conveys some information to the loader,
such as whether the executable needs to run as Administrator or not. This patch
is to parse command line option for manifest file.

Actual XML file generation will be done in a separate patch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193141 91177308-0d34-0410-b5e6-96231b3b80d8