[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`.
Two tests in `sanitizer_type_traits_test.cpp` need to be disabled since
they have undefined references to `__sanitizer::integral_constant<bool,
true>::value`.

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

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