driver: Do not warn about ineffective `-x' option if no inputs were given

Fix an issue with the GCC driver and the `-x' option where a warning is
issued in an invocation like:

$ riscv64-linux-gnu-gcc -print-multi-directory -x c++
riscv64-linux-gnu-gcc: warning: '-x c++' after last input file has no effect
lib64/lp64d
$ 

where no inputs were given and hence the use of `-x' is irrelevant.
The statement printed is also untrue as the `-x' does not come after the
last input file given that none was given.  Do not print it then if no
inputs were supplied.

	* gcc.c (process_command): Only warn about an ineffective `-x'
	option if any input files have actually been supplied.

From-SVN: r278872

gcc/ChangeLog

 2019-11-30  Maciej W. Rozycki  <macro@wdc.com>
 
+	* gcc.c (process_command): Only warn about an ineffective `-x'
+	option if any input files have actually been supplied.
+
+2019-11-30  Maciej W. Rozycki  <macro@wdc.com>
+
 	* doc/install.texi (Options specification): Remove the list of
 	target library subdirectories supporting
 	`--enable-version-specific-runtime-libs'.  Document defaults for

gcc/gcc.c

@@ -4741,7 +4741,7 @@ process_command (unsigned int decoded_options_count,
   /* More prefixes are enabled in main, after we read the specs file
      and determine whether this is cross-compilation or not.  */
 
-  if (n_infiles == last_language_n_infiles && spec_lang != 0)
+  if (n_infiles != 0 && n_infiles == last_language_n_infiles && spec_lang != 0)
     warning (0, "%<-x %s%> after last input file has no effect", spec_lang);
 
   /* Synthesize -fcompare-debug flag from the GCC_COMPARE_DEBUG