Relax assert in ExprConstant to a return None.

Fixes a compiler assert on passing a compile time integer to atomic builtins.

Assert introduced in D61522
Function changed from ->bool to ->Optional in D76646
Simplifies call sites to getIntegerConstantExpr to elide the now-redundant
isValueDependent checks.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D112159