Fix some compilation failures introduced in recent patches.

This fixes two compilation failures:

  1) Designated initializers are C++20.  We can't use them in LLVM.
  2) thread_result_t is not a pointer type on all platforms, so
     returning nullptr is an error.

llvm-svn: 346873
diff --git a/lldb/source/Plugins/Language/ObjC/Cocoa.cpp b/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
index 16f77e8..4aa3e0b 100644
--- a/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
+++ b/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
@@ -783,7 +783,8 @@
   if (encodedTimeInterval == std::numeric_limits<uint64_t>::max())
     return (uint64_t)-0.0;
 
-  TaggedDoubleBits encodedBits = { .i = encodedTimeInterval };
+  TaggedDoubleBits encodedBits = {};
+  encodedBits.i = encodedTimeInterval;
   DoubleBits decodedBits;
 
   // Sign and fraction are represented exactly.