[lldb][NFC] Add 'override' where missing in source/ and tools/

These were found by Clang's new -Wsuggest-override.

This patch doesn't touch any code in unittests/, since much of it intentionally doesn't use override to avoid massive warning spam from -Winconsistent-missing-override due to the use of MOCK_*** macros.

Differential Revision: https://reviews.llvm.org/D83847