[libc++][Modules] Recreate the top level `std` clang module

lldb needs the `std` clang module to make all of libc++ available in the debugger. Make a new header to include the rest of the public headers and use to build a `std` module that just re-exports the rest of libc++.

Reviewed By: Mordante, JDevlieghere, #libc

Differential Revision: https://reviews.llvm.org/D156177