[LLVM-C] Upstream `LLVMGetOrInsertFunction` from rustc (#162235)

Add `LLVMGetOrInsertFunction` to the C API as a thin wrapper over
`Module::getOrInsertFunction`, upstreamed from
[rustc](https://github.com/rust-lang/rust/blob/d773bd07d63a74adcf25ea5f4aae986be94cac5e/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp#L203).
It provides a single-call way to get or create a function declaration,
avoiding `LLVMGetNamedFunction` + `LLVMAddFunction` and is more
idiomatic.

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/unittests/IR/FunctionTest.cpp b/llvm/unittests/IR/FunctionTest.cpp
index 7ba7584..8ed7699 100644
--- a/llvm/unittests/IR/FunctionTest.cpp
+++ b/llvm/unittests/IR/FunctionTest.cpp
@@ -625,4 +625,23 @@ TEST(FunctionTest, Personality) {
   EXPECT_FALSE(LLVMHasPersonalityFn(wrap(F)));
 }
 
+TEST(FunctionTest, LLVMGetOrInsertFunction) {
+  LLVMContext Ctx;
+  Module M("test", Ctx);
+  Type *Int8Ty = Type::getInt8Ty(Ctx);
+  FunctionType *FTy = FunctionType::get(Int8Ty, false);
+
+  // Create the function using the C API
+  LLVMValueRef FuncRef = LLVMGetOrInsertFunction(wrap(&M), "F", 1, wrap(FTy));
+
+  // Verify that the returned value is a function and has the correct type
+  Function *Func = unwrap<Function>(FuncRef);
+  EXPECT_EQ(Func->getName(), "F");
+  EXPECT_EQ(Func->getFunctionType(), FTy);
+
+  // Call LLVMGetOrInsertFunction again to ensure it returns the same function
+  LLVMValueRef FuncRef2 = LLVMGetOrInsertFunction(wrap(&M), "F", 1, wrap(FTy));
+  EXPECT_EQ(FuncRef, FuncRef2);
+}
+
 } // end namespace