[StreamExecutor] Make SE work with an in-tree LLVM build.

Summary:
With these changes, we can put parallel-libs within llvm/projects and
build as normal.

This is kind of the minimal change I could figure out how to make while
still making us compatible with llvm's build system.  Some things I'm
not thrilled about include:

 * The creation of a CoreTests directory (the macros really seemed to
   want this)

 * Pulling SimpleHostPlatformDevice.h into CoreTests.  It seems to me
   this should live inside unittests/include, or maybe tests/include,
   but I didn't want to make that change in this patch.

One important piece of work that remains to be done is to make

  $ ninja check-streamexecutor

run all the tests.  Right now the only way I've figured out to run the
tests is

  $ ninja projects/parallel-libs/streamexecutor/unittests/StreamExecutorUnitTests
  $ projects/parallel-libs/streamexecutor/unittests/CoreTests/CoreTests

Reviewers: jhen

Subscribers: beanz, parallel_libs-commits, jprice

Differential Revision: https://reviews.llvm.org/D24368

llvm-svn: 281091