[clang-format] Look ahead before consuming `bool` in requires clause.

The comment handling the bool case says:
"bool is only allowed if it is directly followed by a paren for a cast"

This change more closely follows this directive by looking ahead for
the paren before consuming the bool keyword itself. Without a following
paren, the bool would be part of something else, such as a return type
for a function declaration

Fixes https://github.com/llvm/llvm-project/issues/57538

Reviewed By: HazardyKnusperkeks, owenpan, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D134325