gcc: Make strchr return value pointers const

This fixes compilation of codepaths for dos-like filesystems
with Clang. When built with clang, it treats C input files as C++
when the compiler driver is invoked in C++ mode, triggering errors
when the return value of strchr() on a pointer to const is assigned
to a pointer to non-const variable.

This matches similar variables outside of the ifdefs for dos-like
path handling.

2020-09-07  Martin Storsjö  <[email protected]>

gcc/
	* dwarf2out.c (file_name_acquire): Make a strchr return value
	pointer to const.
libcpp/
	* files.c (remap_filename): Make a strchr return value pointer
	to const.

Author: mstorsjo

gcc/dwarf2out.c

@@ -12118,7 +12118,7 @@ file_name_acquire (dwarf_file_data **slot, file_name_acquire_data *fnad)
   f = strrchr (f, DIR_SEPARATOR);
 #if defined (DIR_SEPARATOR_2)
   {
-    char *g = strrchr (fi->path, DIR_SEPARATOR_2);
+    const char *g = strrchr (fi->path, DIR_SEPARATOR_2);
 
     if (g != NULL)
       {

libcpp/files.c

@@ -1693,7 +1693,7 @@ remap_filename (cpp_reader *pfile, _cpp_file *file)
       p = strchr (fname, '/');
 #ifdef HAVE_DOS_BASED_FILE_SYSTEM
       {
-	char *p2 = strchr (fname, '\\');
+	const char *p2 = strchr (fname, '\\');
 	if (!p || (p > p2))
 	  p = p2;
       }