[asan] Remove -fsanitize-address-zero-base-shadow command line
flag from clang, and disable zero-base shadow support on all platforms
where it is not the default behavior.

- It is completely unused, as far as we know.
- It is ABI-incompatible with non-zero-base shadow, which means all
objects in a process must be built with the same setting. Failing to
do so results in a segmentation fault at runtime.
- It introduces a backward dependency of compiler-rt on user code,
which is uncommon and complicates testing.

This is the LLVM part of a larger change.
[asan] Remove -fsanitize-address-zero-base-shadow command line
flag from clang, and disable zero-base shadow support on all platforms
where it is not the default behavior.

- It is completely unused, as far as we know.
- It is ABI-incompatible with non-zero-base shadow, which means all
objects in a process must be built with the same setting. Failing to
do so results in a segmentation fault at runtime.
- It introduces a backward dependency of compiler-rt on user code,
which is uncommon and complicates testing.

This is the Clang part of a larger change.