Adds a new frontend option
"-experimental-allow-module-with-compiler-errors". If any compilation
errors occur while generating the .swiftmodule, this mode will skip SIL
entirely and only serialize the (likey invalid) AST.
This existence of this option during generation is serialized into the
resulting .swiftmodule. Errors found in deserialization are only allowed
if it is set.
Primarily intended for IDE requests (eg. indexing and code completion)
to ensure robust cross-module results, despite possible errors.
Resolves rdar://69815975
