Fix std::fpos pretty printer on musl

The mbstate_t field in std::fpos is an opaque type provied by libc,
and musl's implementation does not match the one used by glibc.
Change StdFposPrinter to verify its assumptions about the layout
of mbstate_t, and leave out the state printing if it doesn't match.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D132983