[Reflection] Fail gracefully when objc_debug_taggedpointer_obfuscator isn't present.

Older OSes don't have this variable. We can treat it as 0 when it's not present.

rdar://problem/48411483

Author: mikeash

include/swift/Remote/MetadataReader.h

@@ -2472,8 +2472,13 @@ class MetadataReader {
     TaggedPointerExtendedClasses =
         TaggedPointerExtendedClassesAddr.getAddressData();
 
-    tryFindAndReadSymbol(TaggedPointerObfuscator,
-                         "objc_debug_taggedpointer_obfuscator");
+    // The tagged pointer obfuscator is not present on older OSes, in
+    // which case we can treat it as zero.
+    TaggedPointerObfuscator = 0;
+    auto TaggedPointerObfuscatorAddr = Reader->getSymbolAddress(
+      "objc_debug_taggedpointer_obfuscator");
+    if (TaggedPointerObfuscatorAddr)
+      tryReadSymbol(TaggedPointerObfuscatorAddr, TaggedPointerObfuscator);
 
 #   undef tryFindSymbol
 #   undef tryReadSymbol