`-fsanitize-address-use-after-scope` is the default as of clang 5.0.0,
and we don't allow to enable ASan for older versions anyway.

We no longer try to retrieve the filename of a given stream when
fstat'ing, because this is very slow.  Since we neither didn't do that
in PHP 7.3 and earlier, we regard this as sensible trade-off.

* PHP-7.3:
  Fixed bug #78910
  Fix #78878: Buffer underflow in bc_shift_addsub
  Fix test
  Fix #78862: link() silently truncates after a null byte on Windows
  Fix #78863: DirectoryIterator class silently truncates after a null byte
  Fix #78943: mail() may release string with refcount==1 twice

Since we need `headers_lc` as well as `headers_trim` in the following,
we do not release the former even if they are the same string, to avoid
complicating the release logic even more.

A new test case is not necessary, since we already have
mail_basic_alt2-win32.phpt and others.

* PHP-7.3:
  Fix #78296: is_file fails to detect file

If we're constructing extended-length paths (i.e. paths prefixed with
`\\?\`), we have to replace all forward slashes with backward slashes,
because the former are not supported by Windows for extended-length
paths.

The more efficient and likely cleaner alternative solution would be to
cater to this in `php_win32_ioutil_normalize_path_w()` by always
replacing forward slashes, but that might break existing code.  It
might be sensible to change that for `master`, though.

We must not assume that any file which is not a directory is a regular
file.  Therefore we employ `GetFileType()` in this case to properly
distinguish between character special, FIFO special and regular files.

Checking for the exact linker version appears to be too restrictive; it
should be fine if the tens match.

We also refactor to avoid repeating ourselves.