Limit Options.inc generation to desired targets

[emaste]

Summary

As mentioned in the Makefile there's an atrocious hack to generate a different version of Options.inc.h, depending on the library being built. Remove the catch-all else case and limit it to the two specific libraries, so that we don't accidentally use the Options.inc.h from clangdriver if a future libary also uses Options.inc.h.

Test Plan

make tinderbox

[emaste]

Add clangfrontend and clangfrontendtool, they also use the same Options.inc as clangdriver.

[dim]

Yes, this is a very good safety belt.

[emaste]

Also build Options.inc.h for usr.bin/clang/clang