gdb: avoid unnecessary string copy in auto_load_objfile_script_1

Assigning the result of STRIP_DRIVE_SPEC to an std::string creates an
unnecessary copy of the string.  STRIP_DRIVE_SPEC is defined as:

  #define STRIP_DRIVE_SPEC(f) ((f) + 2)

So if it is passed a "const char *", it returns a "const char *".  We
could use a "const char *" intermediary variable instead of an
std::string, or (as implemented in this patch) just use it directly in
the concatenation right after.

gdb/ChangeLog:

	* auto-load.c (auto_load_objfile_script_1): Don't use
	debugfile_holder as temporary variable when stripping drive
	letter.

Change-Id: If2ccc7a156b22100754d9cdf6778ac7eeb93da4c

gdb/ChangeLog

+2020-10-05  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* auto-load.c (auto_load_objfile_script_1): Don't use
+	debugfile_holder as temporary variable when stripping drive
+	letter.
+
 2020-10-05  Hannes Domani  <ssbssa@yahoo.de>
 	* amd64-windows-tdep.c (amd64_windows_passed_by_integer_register):

gdb/auto-load.c

@@ -777,10 +777,8 @@ auto_load_objfile_script_1 (struct objfile *objfile, const char *realname,
 
       /* Convert Windows file name from c:/dir/file to /c/dir/file.  */
       if (HAS_DRIVE_SPEC (debugfile))
-	{
-	  debugfile_holder = STRIP_DRIVE_SPEC (debugfile);
-	  filename = std::string("\\") + debugfile[0] + debugfile_holder;
-	}
+	filename = (std::string("\\") + debugfile[0]
+		    + STRIP_DRIVE_SPEC (debugfile));
 
       for (const gdb::unique_xmalloc_ptr<char> &dir : vec)
 	{