support %b and %B from WG14 N2630 in format string checking

[enh-google]

glibc already added %b and %B (and apparently GCC knows about this), and i'm adding them to bionic https://android-review.googlesource.com/c/platform/bionic/+/2169037, but needed to use #pragma to shut clang up for the tests.

one interesting gnarl here is that for Android whether %b/%B are allowed or not will depend on the target API level. (similar should be true for %m which went into API level 29, but no-one's ever complained that clang doesn't know that, so it probably doesn't matter?)

[MaskRay]

printf: https://reviews.llvm.org/D131057

scanf is not done. Fortify source is not done.