Introduce module attributes into the module map grammar, along with a

single attribute ("system") that allows us to mark a module as being a
"system" module. Each of the headers that makes up a system module is
considered to be a system header, so that we (for example) suppress
warnings there.

If a module is being inferred for a framework, and that framework
directory is within a system frameworks directory, infer it as a
system framework.

llvm-svn: 149143