This adds detection for missing-save or empty-save, and rebuilds in either case.

Static save files do not require an entry in the save data indexer to mount.
Prior to 17.0.0, save data files were considered static if userid was 0.
In 17.0.0+, only 8000000000000000 is static.

However, some users using cfw do not have an entry for 8000000000000120 in the indexer,
for various reasons (but mostly manual nand-restore, I think). Thus, on boot of 17.0.0+,
FS will say 8000000000000120 is not present (not in indexer), and NCM will create it anew.

The 8000000000000120 save will then be empty, and then the firmware can't boot.

To workaround this, logic has been re-enabled on 17.0.0+ for building the content meta database.
Thus, if the user encounters this error, the 8000000000000120 save will be emptied, but then
it will be automatically reconstructed, fixing the problem.

* result: try out some experimental shenanigans

* result: sketch out some more shenanigans

* result: see what it looks like to convert kernel to use result conds instead of guards

* make rest of kernel use experimental new macro-ing

* 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.