Switch external cvtres.exe for llvm's own resource library.

In this patch, I flip the switch in DriverUtils from using the external
cvtres.exe tool to using the Windows Resource library in llvm.

I also fixed a bug where .rsrc sections were marked as discardable
memory and therefore were placed in the wrong order in the final PE.

Furthermore, I modified WindowsResource to write the coff directly to a
memory buffer instead of to file, also had it use the machine types
already declared in COFF.h instead creating my own enum.

Finally, I flipped the switch to allow all unit tests that had
previously run only on windows due to a winres dependency to run
cross-platform.

Reviewers: zturner, ruiu

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D34265

llvm-svn: 305592