[libc++] Adds more forward declaration headers.

[libc++] Adds more forward declaration headers.

The module validation script of D144994 validate whether the contents of
an include match its module. An include is the set of files matching the
pattern:
- foo
- foo/*.
- __fwd/foo.h

Several declarations of the stream headers are in the header iosfwd.
This gives issue using the validation script. Adding iosfwd to the set
of matching files gives too many declarations. For example when
validating the fstream header it will pull in declarations of the
istream header. Instead if writing a set of filters the headers are
granularized into smaller headers containing the expected declarations.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D148927