[libc++][format] Removes format sources.

The source file is used to anchor the destructor of format_error. When
format is moved from experimental to stable this code would move to the
dylib. One issue with code in the dylib is that it can't be used in
constexpr context. There is a proposal to make format work during
constant evaluation

  P2758 Emitting messages at compile time

This paper has initially been received favourable by EWG. Therefore move
the code to the header. This also avoids possible availability issues on
Mac back deployment targets.

Note it is expected that format will no longer be experimental with the
next LLVM release.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D150073