[Frontend] Fix -MP when main file is <stdin>

rC220726 had a bug: `echo "<cstdlib>" | clang -M -MP -x c++ - 2>/dev/null`
(used by glibc/configure.ac find_cxx_header) omitted a `cstdlib:` line. Instead
of filtering out `<stdin>` in `Dependencies`, retain it (so that the number of
entries does not change whether or not main file is `<stdin>`) and filter the
`PhonyTarget` output.