[clang][cli] Extract FileSystem and Migrator options parsing/generation

This patch splits out the last two option groups (`Filesystem` and `Migrator`) into their own `Parse`/`Generate` functions.

This effectively removes the need for `parseSimpleArgs` and marshalling block in `CompilerInvocation::generateCC1CommandLine`.

The two new `Parse`/`Generate` functions are not part of the round-trip, because they contain no custom code and the very next patch starts round-tripping the whole `CompilerInvocation`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D96278