testsuite: Verify -fshort-enums and -fno-short-enums in pr33738.C

testsuite: Verify -fshort-enums and -fno-short-enums in pr33738.C

For some targets, like Cortex-M on arm-none-eabi, the -fshort-enums is
enabled by default. For these targets, the test case fails as
sizeof(Alpha) < sizeof(int).
To make the test case behave identical for targets that does enable
-fshort-enums and those that does not, add -fno-short-enums in the test
case and verify that the warning is not emitted. Then also create a copy
and run the test with -fshort-enums and verify that the warning is
emitted.

Regtested on x86_64-pc-linux-gnu and arm-none-eabi.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/pr33738.C: Added -fno-short-enums.
	* g++.dg/warn/pr33738-2.C: Duplicate g++.dg/warn/pr33738.C with
	-fshort-enums and removed xfail.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>