[clang-format] Disallow decltype in the middle of constraints

If a function with a `requires` clause as a constraint has a decltype
return type, such as `decltype(auto)`, the decltype was seen to be part
of the constraint clause, rather than as part of the function
declaration, causing it to be placed on the wrong line.

This patch disallows decltype to be a part of these clauses

Fixes https://github.com/llvm/llvm-project/issues/59578

Depends on D140339

Reviewed By: HazardyKnusperkeks, owenpan, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D140312