[exec-ctx] Remove ScopedTimeCache from ExecCtx on iOS (#34416)

Fix a crash on older iOS versions due to problematic thread-local
variable initialization.

See firebase/firebase-ios-sdk#11509

Basically, there appears to be a bug in Xcode where it generates
assembly for thread-local variable initialization that is susceptible to
a crash. For example, on arm64 the generated assembly relies on
registers like x8 and x10 being preserved by the thread-local variable
initialization routine; however, in some cases this thread-local
variable initialization calls functions like
`ImageLoaderMachOCompressed::doBindFastLazySymbol` which clobber these
registers, leaving their values indeterminate when the caller resumes.
When those indeterminate values are later used as memory addresses they
are invalid and result in a crash.

This PR works around this bug by removing the `ScopedTimeCache` member
variable from the `ExecCtx` class on iOS. This is a reasonable
workaround because `ScopedTimeCache` is only a slight optimization for
data centers that entirely doesn't matter for mobile.

See https://github.com/dconeybe/TlsCrashIos12 for a demo of this crash.

Googlers see b/300501963 for full details.

src/core/lib/iomgr/exec_ctx.h

@@ -23,6 +23,11 @@
 
 #include <limits>
 
+#if __APPLE__
+// Provides TARGET_OS_IPHONE
+#include <TargetConditionals.h>
+#endif
+
 #include <grpc/impl/grpc_types.h>
 #include <grpc/support/atm.h>
 #include <grpc/support/cpu.h>
@@ -180,13 +185,26 @@ class GRPC_DLL ExecCtx {
   void SetReadyToFinishFlag() { flags_ |= GRPC_EXEC_CTX_FLAG_IS_FINISHED; }
 
   Timestamp Now() { return Timestamp::Now(); }
-  void InvalidateNow() { time_cache_.InvalidateCache(); }
+
+  void InvalidateNow() {
+#if !TARGET_OS_IPHONE
+    time_cache_.InvalidateCache();
+#endif
+  }
+
   void SetNowIomgrShutdown() {
+#if !TARGET_OS_IPHONE
     // We get to do a test only set now on this path just because iomgr
     // is getting removed and no point adding more interfaces for it.
     time_cache_.TestOnlySetNow(Timestamp::InfFuture());
+#endif
+  }
+
+  void TestOnlySetNow(Timestamp now) {
+#if !TARGET_OS_IPHONE
+    time_cache_.TestOnlySetNow(now);
+#endif
   }
-  void TestOnlySetNow(Timestamp now) { time_cache_.TestOnlySetNow(now); }
 
   /// Gets pointer to current exec_ctx.
   static ExecCtx* Get() { return EXEC_CTX; }
@@ -211,7 +229,9 @@ class GRPC_DLL ExecCtx {
   CombinerData combiner_data_ = {nullptr, nullptr};
   uintptr_t flags_;
 
+#if !TARGET_OS_IPHONE
   ScopedTimeCache time_cache_;
+#endif
 
 #if !defined(_WIN32) || !defined(_DLL)
   static thread_local ExecCtx* exec_ctx_;