[clang-format] Change heuristic for locating lambda template arguments

Previously, the heuristic was simply to look for template argument-
specific keywords, such as typename, class, template and auto
that are preceded by a left angle bracket <.

This changes the heuristic to instead look for a left angle bracket <
preceded by a right square bracket ], since according to the C++
grammar, the template arguments must *directly* follow the introducer.
(This sort of check might just end up being *too* aggressive)

This patch also adds a bunch more token annotator tests for lambdas,
specifically for some of the stranger forms of lambdas now allowed as
of C++20 or soon-to-be-allowed as part of C++23.

Fixes https://github.com/llvm/llvm-project/issues/57093

This does NOT resolve the FIXME regarding explicit template lists, but
perhaps it gets closer

Differential Revision: https://reviews.llvm.org/D132295