Teach clang that -no-pthread is a valid command line option

The winpthreads library in mingw-w64 passes -no-pthread when building
since pthreads is not available to build itself and pthreads it is linked 
by default. clang does not link to pthreads by default but did error on 
unknown -no-pthread option thus stopping the winpthreads build.

http://reviews.llvm.org/D11087

Patch by Martell Malone.

llvm-svn: 241929