[clang-tidy] Fix crashes on `if consteval` in readability checks

The `readability-braces-around-statements` check tries to look at the
closing parens of the if condition to determine where to insert braces,
however, "consteval if" statements don't have a condition, and always
have braces regardless, so the skip can be checked.

The `readability-simplify-boolean-expr` check looks at the condition
of the if statement to determine what could be simplified, but as
"consteval if" statements do not have a condition that could be
simplified, they can also be skipped here.

There may still be more checks that try to look at the conditions of
`if`s that aren't included here

Fixes https://github.com/llvm/llvm-project/issues/57568

Reviewed By: njames93, aaron.ballman

Differential Revision: https://reviews.llvm.org/D133413