[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.

llvm-svn: 199372

clang/lib/Frontend/CompilerInvocation.cpp

@@ -452,8 +452,6 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.SanitizerBlacklistFile = Args.getLastArgValue(OPT_fsanitize_blacklist);
   Opts.SanitizeMemoryTrackOrigins =
     Args.hasArg(OPT_fsanitize_memory_track_origins);
-  Opts.SanitizeAddressZeroBaseShadow =
-    Args.hasArg(OPT_fsanitize_address_zero_base_shadow);
   Opts.SanitizeUndefinedTrapOnError =
     Args.hasArg(OPT_fsanitize_undefined_trap_on_error);
   Opts.SSPBufferSize =