Add __has_feature(modules) to indicate when modules are available (in

any language variant), and restrict __has_feature(objc_modules) to
mean that we also have the Objective-C @import syntax. I anticipate
__has_feature(cxx_modules) and/or __has_feature(c_modules) for when we
nail down the module syntax for C/C++.

llvm-svn: 147548