swift234

[CMake] Derive module names from target names rather than source file names. (#4494)

This will remove the restriction that overlays always need to have a source
file matching the name of the framework, at the "cost" of standardizing the
"swiftFoo" convention for libraries with a module named "Foo". (Until
someone exposes the MODULE_NAME setting at the add_swift_library level.)

If this passes all tests, a follow-up commit will adjust the names of
some of the source files in stdlib/.

rdar://problem/17535693