Improve gdb.ada/rename_subscript_param.exp by using more unique names.

Improve gdb.ada/rename_subscript_param.exp by using more unique names.

With old compilers, the test fails because no debug info is generated
for 'B' and GDB finds some 'b' in atnat.h:

(gdb) print b
Multiple matches for b
[0] cancel
[1] b at ../sysdeps/ieee754/dbl-64/atnat.h:106
[2] b at ../sysdeps/ieee754/dbl-64/atnat.h:106
[3] b at ../sysdeps/ieee754/dbl-64/atnat.h:106
> FAIL: gdb.ada/rename_subscript_param.exp: print b before changing its value (timeout)

Avoid the timeout by renaming 'b' to rename_subscript_param_b.

Also, change 'before' to 'after' in the gdb_test message that prints
the value after changing it.

The test still fails with old compilers that do not properly
generate debug info for this renaming:
(gdb) print rename_subscript_param_b
No definition of "rename_subscript_param_b" in current context.
(gdb) FAIL: gdb.ada/rename_subscript_param.exp: print rename_subscript_param_b before changing its value

Note: if the compiler would generate the correct debug info, the test should
succeed with the name B.  However, waiting for this fix, changing the name
ensures that the test fails directly, instead of causing a timeout.

2018-12-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	PR ada/23381
	* gdb.ada/rename_subscript_param/pkg.adb (B): Rename to
	Rename_Subscript_Param_B.  All users updated.
	gdb.ada/rename_subscript_param.exp: Test names made unique.
	Note that PR ada/23381 is only fully fixed when using a recent
	compiler.