Adds a modified options parser (shamefully pulled from Scudo, which
shamefully pulled it from sanitizer-common) to GWP-ASan. This allows
customers (Android) to parse options strings in a common way.

Depends on D94117.

AOSP side of these patches is staged at:

 - sepolicy (sysprops should only be settable by the shell, in both root and
 unrooted conditions):
 https://android-review.googlesource.com/c/platform/system/sepolicy/+/1517238

 - zygote updates:
 https://android-review.googlesource.com/c/platform/frameworks/base/+/1515009

 - bionic changes to add `gwp_asan.<process_name>` system property, and
 GWP_ASAN_OPTIONS environment variable:
 https://android-review.googlesource.com/c/platform/bionic/+/1514989

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D92696

Summary:
@eugenis to approve addition of //compiler-rt/tools.
@pree-jackie please confirm that this WFY.

D66494 introduced the GWP-ASan stack_trace_compressor_fuzzer. Building fuzz
targets in compiler-rt is a new affair, and has some challenges:
- If the host compiler doesn't have compiler-rt, the -fsanitize=fuzzer may not
  be able to link against `libclang_rt.fuzzer*`.
- Things in compiler-rt generally aren't built when you want to build with
  sanitizers using `-DLLVM_USE_SANITIZER`. This tricky to work around, so
  we create the new tools directory so that we can build fuzz targets with
  sanitizers. This has the added bonus of fixing the problem above as well, as
  we can now just guard the fuzz target build to only be done with
  `-DLLVM_USE_SANITIZE_COVERAGE=On`.

Reviewers: eugenis, pree-jackie

Reviewed By: eugenis, pree-jackie

Subscribers: dberris, mgorny, #sanitizers, llvm-commits, eugenis, pree-jackie, lebedev.ri, vitalybuka, morehouse

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D66776

llvm-svn: 370094