[ADT][NFC] Expose fltSemantics struct (#175676)
This patch moves the `fltSemantics` struct definition (along with
`fltNonfiniteBehavior`
and `fltNanEncoding` enums) from APFloat.cpp to APFloat.h, making them
part of the
public API.
Currently, downstream projects cannot define custom floating-point
semantics because
`fltSemantics` is an opaque forward declaration in the header. This
forces projects
with specialized float formats to either patch LLVM locally or request
new formats be added upstream for each variant. By exposing the struct,
downstream users can define their own semantics.
