[Modules] Add 'freestanding' to the 'requires-declaration' feature-list.

This adds support for modules that require (non-)freestanding
environment, such as the compiler builtin mm_malloc submodule.

Differential Revision: https://reviews.llvm.org/D23871

llvm-svn: 280613
4 files changed