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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@367676 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm