Sanitizers don't fall back to the `LLVM_SYMBOLIZER_PATH` env variable

[chandlerc]

Currently, my understanding is that the sanitizer runtimes look for an external symbolizer with ASAN_SYMBOLIZER_PATH (or UBSAN_..., ... depending on the sanitizer), and if it isn't found looks for llvm-symbolizer binary in the PATH.

It would be nice to teach it to look for LLVM_SYMBOLIZER_PATH environment variable after the sanitizer-specific environment variable and prior to looking in the PATH. Lots of systems seem to spend quite a bit of time basically replicating the path of the symbolizer binary across N different environment variables because this is missing, including Lit for example.