Add debuginfo-tests that use cdb on Windows

This is an initial prototype of how we can run debugger integration
tests on Windows. cdb and windbg share a command language and debugger
engine. Visual Studio has its own, but we should at least be able to use
cdb as the basis for optimized debug info integration tests.

There's a lot of work to do here still. For example:
- Make fewer assumptions about the SDK location
- Don't assume x64 (important, I need x86 testing)
- More environment isolation, have lit setup vcvars instead of passing
  LIB and INCLUDE down.
- Write a .py file to replace the grep+sed RUN line

But, this seemed like a good enough concept to commit as is, since it's
useful to me already.

Reviewers: aprantl, zturner

Differential Revision: https://reviews.llvm.org/D54187

llvm-svn: 361889

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