[modules] Flatten -fmodule-name= and -fmodule-implementation-of= into a single

option. Previously these options could both be used to specify that you were
compiling the implementation file of a module, with a different set of minor
bugs in each case.

This change removes -fmodule-implementation-of, and instead tracks a flag to
determine whether we're currently building a module. -fmodule-name now behaves
the same way that -fmodule-implementation-of previously did.

llvm-svn: 261372