Reland "[compiler-rt][test] Heed COMPILER_RT_DEBUG when compiling unittests"

When trying to debug some `compiler-rt` unittests, I initially had a hard
time because

- even in a `Debug` build one needs to set `COMPILER_RT_DEBUG` to get
  debugging info for some of the code and
- even so the unittests used a hardcoded `-O2` which often makes debugging
  impossible.

This patch addresses this by instead using `-O0` if `COMPILER_RT_DEBUG`.

Changes relative to the previous commit:

- Use `string(APPEND)` for `COMPILER_RT_TEST_COMPILER_CFLAGS`.
- Omit `-O3` from `COMPILER_RT_TEST_COMPILER_CFLAGS` in non-debug builds for now.
- Provide `__sanitizer::integral_constant<bool, true>::value` instantiation
  for `sanitizer_type_traits_test.cpp` in debug builds.
- Disable subtests of `tsan/tests/unit/tsan_trace_test.cpp` that deadlock
  in debug builds.
- `XFAIL` `tsan/Linux/check_memcpy.c` in debug builds.

Tested on `sparcv9-sun-solaris2.11`, `amd64-pc-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D91620