[Bug pch/78970] GCC crashes if input file is dash

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78970

--- Comment #7 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
This change causes a regression testing glibc.  glibc uses "gcc -E -x 
c-header -", which now results in an error "cannot use '-' as input 
filename for a precompiled header".  But that use with -E is not creating 
a precompiled header, so should not result in an error.