This provides further speed-ups for TUs with many test cases templated
over the same set of types. For especially test-heavy (but still
realistic) TUs, the speed up is up to 20%.

This provides small (but measurable) improvements in compilation
times when a TU has multiple test cases templated over the same types.

We used to store a full vector of "views" of the registered test infos,
so that it can be retrieved externally. However, the only user of that
interface left was `Session`, specifically `Session::applyFilenameAsTags`.

By moving the adding of the filename tags into the `ITestCaseRegistry`
interface, we avoid storing the vector and make the initial test registration
faster.

The new code is ugly and shows a design/layering issue, but no worse than
the previous approach did.

Preallocating to some reasonable and small number of tests avoids
much of the geometric-reallocation threashing at low sizes, without
taking up too much memory for tiny test binaries.

* fix: work around clang 20/21 + libc++ compile failure in TEMPLATE_PRODUCT_TEST_CASE with differing arities

Fixes #3115

* docs: reference llvm/llvm-project#130778 in the clang 20/21 workaround comment

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

This provides nice speedup when writing strings that don't need
escaping, at about 6% in Debug build and ~40% in Release build.
If the strings do need escaping the speedup is much smaller, at
~1% in Debug and ~6% in Release build, as the cost of actually
escaping the strings dwarves the cost of checking.

In practice, the majority of values the JSON writer handles are
strings, but this makes the code more obvious and can still provide
a small speed-up.