[libc++] Granularize the <new> header (#119270)

This disentangles the code which previously had a mix of many #ifdefs, a
non-versioned namespace and a versioned namespace. It also makes it
clearer which parts of <new> are implemented on Windows by including <new.h>.

GitOrigin-RevId: 9474e09459189fbed30f329a669f9c14979c5367
1 file changed