[llvm-dlltool] Clarify an error message. NFC.

The parameter to the -D (--dllname) option is the name of the dll
that llvm-dlltool produces an import library for. Even though this
is named "OutputFile" in the COFFModuleDefinition class, it's not
an output file name in the context of llvm-dlltool, but the name
of the DLL to create an import library for.

llvm-svn: 367676