[Sema] Recognize format argument indicated by format attribute inside blocks
- `[[format(archetype, fmt-idx, ellipsis)]]` specifies that a function accepts a
format string and arguments according to `archetype`. This is how Clang
type-checks `printf` arguments based on the format string.
- Clang has a `-Wformat-nonliteral` warning that is triggered when a function
with the `format` attribute is called with a format string that is not
inspectable because it isn't constant. This warning is suppressed if the
caller has the `format` attribute itself and the format argument to the callee
is the caller's own format parameter.
- When using the `format` attribute on a block, Clang wouldn't recognize its
format parameter when calling another function with the format attribute. This
would cause unsuppressed -Wformat-nonliteral warnings for no supported reason.
Reviewed By: ahatanak
Differential Revision: https://reviews.llvm.org/D112569
Radar-Id: rdar://
84603673