[RFC][debuginfo-test] Rename debug-info lit tests for general purposes

Discussion thread:
https://lists.llvm.org/pipermail/llvm-dev/2021-January/148048.html

Move debuginfo-test into a subdirectory of a new top-level directory,
called cross-project-tests. The new name replaces "debuginfo-test" as an
LLVM project enabled via LLVM_ENABLE_PROJECTS.

Differential Revision: https://reviews.llvm.org/D95339

Reviewed by: aprantl