NOTE: This work is not yet fully complete; kernel is done, but
it was taking an exceedingly long time to get through libstratosphere.
Thus, I've temporarily added -Wno-error=unused-result for libstratosphere/stratosphere.

All warnings should be fixed to do the same thing Nintendo does as relevant, but this
is taking a phenomenally long time and is not actually the most important work to do,
so it can be put off for some time to prioritize other tasks for 21.0.0 support.

* Work around Clang's incomplete C++20 support for omitting typename

* vapours: fix Clang error about missing return in constexpr function

* stratosphere: fix call to non-constexpr strlen in constexpr function

strlen being constexpr is a non-compliant GCC extension; Clang
explicitly rejects it: https://reviews.llvm.org/D23692

* stratosphere: add a bunch of missing override specifiers

* stratosphere: work around Clang consteval bug

Minimal example: https://godbolt.org/z/MoM64v93M

The issue seems to be that Clang does not consider f(x) to be a
constant expression if x comes from a template argument that isn't
a non-type auto template argument (???)

We can work around this by relaxing GetMessageHeaderForCheck (by using
constexpr instead of consteval). This produces no functional changes
because the result of GetMessageHeaderForCheck() is assigned to a
constexpr variable, so the result is guaranteed to be computed
at compile-time.

* stratosphere: fix missing require clauses in definitions

GCC not requiring the require clauses to be repeated for member
definitions is actually a compiler bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96830

Clang rejects declarations with missing require clauses.

* Fix ALWAYS_INLINE_LAMBDA and parameter list relative order

While GCC doesn't seem to care about the position of the always_inline
attribute relative to the parameter list, Clang is very picky
and requires the attribute to appear after the parameter list
(and before a trailing return type)

* stratosphere: fix static constexpr member variable with incomplete type

GCC accepts this for some reason (because of the lambda?) but Clang
correctly rejects this.

This caught an embarrassingly large number of bugs.