testsuite, cp: add expected failures to pass-by-ref tests for certain compilers

testsuite, cp: add expected failures to pass-by-ref tests for certain compilers

There exist expected failures in the pass-by-ref.exp and
pass-by-ref-2.exp tests based on the GCC and Clang version.

* GCC version <= 6 and Clang do not emit DW_AT_deleted and
  DW_AT_defaulted.

* Clang version >= 7 emits DW_AT_calling_convention, which helps the
  debugger make the right calling convention decision in some cases
  despite lacking the 'defaulted' and 'deleted' attributes.

Mark the related tests as XFAIL based on the compiler version.

Tested on X86_64 using GCC 5.5.0, 6.5.0, 7.4.0, 8.3.0, 9.2.1;
and Clang 5.0.1, 6.0.0, 7.0.0, 8.0.0, 9.0.1, 10.0.0.

gdb/testsuite/ChangeLog:
2020-01-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.cp/pass-by-ref-2.exp: Mark some tests as XFAIL based on the
	GCC/Clang version.
	* gdb.cp/pass-by-ref.exp: Ditto.

Change-Id: I1d8440aa438049f7c4da7f4f76f201c48550f1e4