Add support for musl-libc on Linux

[Joerg Sonnenberger via cfe-commits]

On Sun, Jun 12, 2016 at 10:51:11AM +0800, Lei Zhang via llvm-commits wrote:
> Hi,
> 
> I'm replying to this thread; sorry I wasn't subscribed to the list,
> thus cannot reply to it directly.
> 
> http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160606/161733.html
> 
> Joerg, thanks for your reply. Could you please tell me what kind of
> test cases I should prepare?

The target/triple parser has a unit test in
unittests/ADT/TripleTest.cpp. The rest should get output validation in
clang's test/Driver directory. Not sure which one is the primary
GNU/Linux test.

Joerg