Add diagnostic option backing field for -fansi-escape-codes

Summary:
Keep track of -fansi-escape-codes in DiagnosticOptions and move the
option to the new option parsing system.

Depends on D82860

Reviewers: Bigcheese

Subscribers: dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82874