[ConstExprPreter] Removed the flag forcing the use of the interpreter

Summary:
Removed the ```-fforce-experimental-new-constant-interpreter flag```, leaving
only the ```-fexperimental-new-constant-interpreter``` one. The interpreter
now always emits an error on an unsupported feature.

Allowing the interpreter to bail out would require a mapping from APValue to
interpreter memory, which will not be necessary in the final version. It is
more sensible to always emit an error if the interpreter fails.

Reviewers: jfb, Bigcheese, rsmith, dexonsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70071