[modules] Teach the AST reader to handle the case of importing a module

with a subset of the existing target CPU features or mismatched CPU
names.

While we can't check that the CPU name used to build the module will end
up being able to codegen correctly for the translation unit, we actually
check that the imported features are a subset of the existing features.

While here, rewrite the code to use std::set_difference and have it
diagnose all of the differences found.

Test case added which walks the set relationships and ensures we
diagnose all the right cases and accept the others.

No functional change for implicit modules here, just better diagnostics.

llvm-svn: 232248