[Clang] Enable -fsanitize=leak on Fuchsia targets

This required some fixes to the generic code for two issues:

1. -fsanitize=safe-stack is default on x86_64-fuchsia and is *not* incompatible with -fsanitize=leak on Fuchisa
2. -fsanitize=leak and other static-only runtimes must not be omitted under -shared-libsan (which is the default on Fuchsia)

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D73397