libcpp/remap: Only override if string matched

For systems with HAVE_DOS_BASED_FILE_SYSTEM set, only override the
pointer if the backslash pattern matches.

Output without this patch:
.../gcc/testsuite/gcc.dg/cpp/pr71681-2.c:5:10: fatal error: a/t2.h: No such file or directory

With patch applied, no output and the test case succeeds.

libcpp/ChangeLog

	* files.cc: Ensure pattern matches before use.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>