Reapply "[clang][cli] Port DiagnosticOpts to new option parsing system"

This reverts commit 8e3e148c

This commit fixes two issues with the original patch:
* The sanitizer build bot reported an uninitialized value. This was caused by normalizeStringIntegral not returning None on failure.
* Some build bots complained about inaccessible keypaths. To mitigate that, "this->" was added back to the keypath to restore the previous behavior.