Use -qualified flag when setting temporary breakpoint in start command

When using the "start" command, GDB puts a temporary breakpoint on the
"main" symbol (we literally invoke the tbreak command).  However, since
it does wild matching by default, it also puts a breakpoint on any C++
method or "main" function in a namespace.  For example, when debugging
GDB, it creates a total of 24 locations:

  (gdb) start
  Temporary breakpoint 1 at 0x198c1e9: main. (24 locations)

as there are a bunch of methods called main in the selftests, such as

  selftests::string_view::capacity_1::main()

If such method was called in the constructor of a global object, or a
function marked with the attribute "constructor", then we would stop at
the wrong place.  Also, this causes a few extra symtabs (those that
contain the "wrong" mains) to be expanded for nothing.

The dummiest, most straightforward solution is to add -qualified when
invoking tbreak.  With this patch, "start" creates a single-location
breakpoint, as expected.

I copied the start.exp test to start-cpp.exp and made it use a C++ test
file, which contains two main functions.  The new test verifies that the
output of "start" is the output we get when we set a single-location
breakpoint.

gdb/ChangeLog:

	* infcmd.c (run_command_1): Pass -qualified to tbreak when usind
	the "start" command.

gdb/testsuite/ChangeLog:

	* gdb.base/start-cpp.exp: New file.
	* gdb.base/start-cpp.cc: New file.

gdb/infcmd.c

@@ -604,7 +604,10 @@ run_command_1 (const char *args, int from_tty, enum run_how run_how)
 
   /* Insert temporary breakpoint in main function if requested.  */
   if (run_how == RUN_STOP_AT_MAIN)
-    tbreak_command (main_name (), 0);
+    {
+      std::string arg = string_printf ("-qualified %s", main_name ());
+      tbreak_command (arg.c_str (), 0);
+    }
 
   exec_file = get_exec_file (0);