[Modules] Add platform and environment features to requires clause

Allows module map writers to add build requirements based on
platform/os. This helps when target features and language dialects
aren't enough to conditionalize building a module, among other things,
it allow module maps for different platforms to live in the same file.

rdar://problem/43909745

Differential Revision: https://reviews.llvm.org/D51910

llvm-svn: 342499