[Orc] Re-add C bindings for the Orc APIs, with a fix to remove the union that
was causing builder failures.

The bindings were originally added in r251472, and reverted in r251473 due to
the builder failures.

llvm-svn: 251482
diff --git a/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.cpp b/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.cpp
index 5fea3c8..1b5485d 100644
--- a/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.cpp
+++ b/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.cpp
@@ -15,6 +15,8 @@
 
 using namespace llvm;
 
+bool OrcExecutionTest::NativeTargetInitialized = false;
+
 ModuleBuilder::ModuleBuilder(LLVMContext &Context, StringRef Triple,
                              StringRef Name)
   : M(new Module(Name, Context)),