D109833 makes the flags `--builder` and `--binary` mutually exclusive, which
caused some regression tests to fail. Add a new substitution
`%dexter_regression_base` that doesn't include the `--builder`, `--cflags` or
`--ldflags` flags and use that for tests that use the `--binary` flag.
Reviewed By: jmorse
Differential Revision: https://reviews.llvm.org/D112624
Also mark debuginfo_tests as UNSUPPORTED if clang can't be found and
remove it from the list of test dependencies if not in
LLVM_ENABLE_PROJECTS.
Differential Revision: https://reviews.llvm.org/D96511
Reviewed by: aprantl