Revert "[lldb][NFC] Change FindDefinitionTypeForDWARFDeclContext() to take DWARFDIE"

The changes in https://reviews.llvm.org/D138612 cause a testsuite
failure on Darwin systems in TestCPPAccelerator.py, first flagged
by the "LLDB Incremental" CI bot.
https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/
with the specific failure text appended to the phabracator.

This reverts commit c3c423b6.