COFF: Support /manifestinput command line option.

Manifest file is a separate or embedded XML file having metadata
of an executable. As it is XML, it can contain various types of
information. Probably the most popular one is to request escalated
priviledges.

Usually the linker creates an XML file and embed that file into
an executable. However, there's a way to supply an XML file from
command line. /manifestniput is it.

Apparently it is over-designed here, but if you supply two or more
manifest files, then the linker needs to merge the files into a
single XML file. A good news is that we don't need to do that ourselves.
MT.exe command can do that, so we call the command from the linker
in this patch.

llvm-svn: 266704