[unittest][DebugInfo/DWARF] Check that dwarfgen::Generator is created

If Generator::create() returns an error, tests should fail gracefully
and report the cause, for example:

[ RUN      ] DebugLineBasicFixture.ParserSkipsCorrectly
.../llvm/unittests/DebugInfo/DWARF/DWARFDebugLineTest.cpp:47: Failure
Value of: llvm::detail::TakeExpected(ExpectedGenerator)
Expected: succeeded
  Actual: failed  (no asm backend for target nvptx64-nvidia-cuda)

Differential Revision: https://reviews.llvm.org/D116106