[Modules] Be consistent about finding a module for framework headers

We use findModuleForHeader() in several places, but in header search we
were not calling it when a framework module didn't show up with the
expected name, which would then lead to unexpected non-modular includes.
Now we will find the module unconditionally for frameworks.  For regular
frameworks, we use the spelling of the module name from the module map
file, and for inferred ones we use the canonical directory name.

In the future we might want to lock down framework modules sufficiently
that these name mismatches cannot happen.

rdar://problem/20465870

llvm-svn: 241258