[clang-format] Annotate lambdas with requires clauses.

The C++ grammar allows lambdas to have a *requires-clause* in two
places, either directly after the *template-parameter-list*, such as:

`[] <typename T> requires foo<T> (T t) { ... };`

Or, at the end of the *lambda-declarator* (before the lambda's body):

`[] <typename T> (T t) requires foo<T> { ... };`

Previously, these cases weren't handled at all, resulting in weird
results.

Note that this commit only handles token annotation, so the actual
formatting still ends up suboptimal. This is mostly because I do not yet
know how to approach making the requires clause formatting of lambdas
match the formatting for functions.

Fixes https://github.com/llvm/llvm-project/issues/61269

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D145642