jit: Supply LLVMGlobalGetValueType() for LLVM < 8.

Commit 37d5babb used this C API function while adding support for LLVM
16 and opaque pointers, but it's not available in LLVM 7 and older.
Provide it in our own llvmjit_wrap.cpp.  It just calls a C++ function
that pre-dates LLVM 3.9, our minimum target.

Back-patch to 12, like 37d5babb.

Discussion: https://postgr.es/m/CA%2BhUKGKnLnJnWrkr%3D4mSGhE5FuTK55FY15uULR7%3Dzzc%3DwX4Nqw%40mail.gmail.com

src/backend/jit/llvm/llvmjit_wrap.cpp

@@ -88,3 +88,11 @@ LLVMGetFunctionType(LLVMValueRef r)
 {
 	return llvm::wrap(llvm::unwrap<llvm::Function>(r)->getFunctionType());
 }
+
+#if LLVM_VERSION_MAJOR < 8
+LLVMTypeRef
+LLVMGlobalGetValueType(LLVMValueRef g)
+{
+	return llvm::wrap(llvm::unwrap<llvm::GlobalValue>(g)->getValueType());
+}
+#endif

src/include/jit/llvmjit.h

@@ -140,6 +140,10 @@ extern unsigned LLVMGetAttributeCountAtIndexPG(LLVMValueRef F, uint32 Idx);
 extern LLVMTypeRef LLVMGetFunctionReturnType(LLVMValueRef r);
 extern LLVMTypeRef LLVMGetFunctionType(LLVMValueRef r);
 
+#if LLVM_MAJOR_VERSION < 8
+extern LLVMTypeRef LLVMGlobalGetValueType(LLVMValueRef g);
+#endif
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif