[Test][DebugInfo] Check for backend object emission support.

The XCore backend does not support object emission. Several tests fail for this
reason when XCore is the default target. See staging buildbot builder:
clang-xcore-ubuntu-20-x64.

So check for backend object emission before running the tests requiring it.

Incorporate isConfigurationSupported functionality in isObjectEmissionSupported,
to avoid calling them both in the same tests.

Differential Revision: https://reviews.llvm.org/D98400