[clang-format] Don't require deduction guides to be templates

The C++ standard doesn't require that class template deduction guides
be templates themselves, but previously `isDeductionGuide` would assert
for the existence of a template closer or requires clause closer before
the deduction guide declaration.

This patch simply removes that check. Because of this, a test which
asserted that `x()->x<1>;` *isn't* a deduction guide failed, as it is
now formatted as a deduction guide. However, as @JohelEGP demonstrated,
it is [possible to make that a viable deduction guide][1].

Thus, that test has been removed, but other tests related to
non-template class template deduction guides have been added.

Fixes https://github.com/llvm/llvm-project/issues/55142

[1]: https://compiler-explorer.com/z/Wx3K6d5K9

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D139416