Fix TestInlines.py on Windows
The inlining semantics for C and C++ are different, which affects the test's expectation of the number of times the function should appear in the binary. In the case of this test, C semantics means there should be three instances of inner_inline, while C++ semantics means there should be only two.
On Windows, clang uses C++ inline semantics even for C code, and there doesn't seem to be a combination of compiler flags to avoid this.
So, for consistency, I've recast the test to use C++ everywhere. Since the test resided under lang/c, it seemed appropriate to move it to lang/cpp.
This does not address the other XFAIL for this test on Linux/gcc. See https://llvm.org/bugs/show_bug.cgi?id=26710
Differential Revision: http://reviews.llvm.org/D17650