[clang] Implement __attribute__((format_matches)) (#116708)

This implements ``__attribute__((format_matches))``, as described in the
RFC:
https://discourse.llvm.org/t/rfc-format-attribute-attribute-format-like/83076

The ``format`` attribute only allows the compiler to check that a format
string matches its arguments. If the format string is passed
independently of its arguments, there is no way to have the compiler
check it. ``format_matches(flavor, fmtidx, example)`` allows the
compiler to check format strings against the ``example`` format string
instead of against format arguments. See the changes to AttrDocs.td in
this diff for more information.

Implementation-wise, this change subclasses CheckPrintfHandler and
CheckScanfHandler to allow them to collect specifiers into arrays, and
implements comparing that two specifiers are equivalent.
`checkFormatStringExpr` gets a new `ReferenceFormatString` argument that
is piped down when calling a function with the `format_matches`
attribute (and is `nullptr` otherwise); this is the string that the
actual format string is compared against.

Although this change does not enable -Wformat-nonliteral by default,
IMO, all the pieces are now in place such that it could be.

0 files changed, 0 insertions, 0 deletions