Disable "UseOdrIndicator" ASan instrumentation mode by default.

Previously Swift enabled the "UseOdrIndicator" ASan instrumentation mode
and gave no option to disable this. This probably wasn't intentional but
happened due to the fact the
`createModuleAddressSanitizerLegacyPassPass()` function has a default
value for the `UseOdrIndicator` parameter of `true` and in Swift we
never specified this parameter explicitly.

Clang disables the "UseOdrIndicator" mode by default but allows it to be
enabled using the `-fsanitize-address-use-odr-indicator` flag.
Having "UseOdrIndicator" off by default is probably the right
default choice because it bloats the binary. So this patch changes the
Swift compiler to match Clang's behavior.

This patch disables the "UseOdrIndicator" mode by default but adds a
hidden driver and frontend flag (`-sanitize-address-use-odr-indicator`)
to enable it. The flag is hidden so that we can remove it in the future
if needed.

A side effect  of disabling "UseOdrIndicator" is that by we will no
longer use private aliases for poisoning globals. Private aliases were
introduced to avoid crashes
(https://github.com/google/sanitizers/issues/398) due to ODR violations
with non-instrumented binaries. On Apple platforms the use of two-level
namespaces probably means that using private aliases wasn't ever really
necessary to avoid crashes. On platforms with a flat linking namespace
(e.g. Linux) using private aliases might matter more but should users
actually run into problems they can either:

* Fix their environment to remove the ODR, thus avoiding the crash.
* Instrument the previously non-instrumented code to avoid the crash.
* Use the new `-sanitize-address-use-odr-indicator` flag

rdar://problem/69335186

lib/Frontend/CompilerInvocation.cpp

@@ -1228,6 +1228,12 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
                                        /*emitWarnings=*/true);
   }
 
+  if (const Arg *A =
+          Args.getLastArg(options::OPT_sanitize_address_use_odr_indicator)) {
+    IRGenOpts.SanitizeAddressUseODRIndicator =
+        parseSanitizerAddressUseODRIndicator(A, Opts.Sanitizers, Diags);
+  }
+
   if (auto A = Args.getLastArg(OPT_enable_verify_exclusivity,
                                OPT_disable_verify_exclusivity)) {
     Opts.VerifyExclusivity