Allow a new syntax in a module requires-declaration:

Allow a new syntax in a module requires-declaration:

  requires ! feature

The purpose of this is to allow (for instance) the module map for /usr/include
to exclude <tgmath.h> and <complex.h> when building in C++ (these headers are
instead provided by the C++ standard library in this case, and the glibc C
<tgmath.h> header would otherwise try to include <complex.h>, resulting in a
module cycle).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193549 91177308-0d34-0410-b5e6-96231b3b80d8