[CIR][LLVMLowering] Add LLVM lowering support for _Float16 (llvm#716)

This PR adds LLVM lowering support for `_Float16` type. The only change
we need to make here is adding a new type converter to the LLVM lowering
pass. The majority of this PR is tests that check the generated LLVM IR.

Later I'll add another separate PR that adds LLVM lowering support for
the `__bf16` type. This PR is already big enough.

Author: Lancern

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -3420,6 +3420,9 @@ void prepareTypeConverter(mlir::LLVMTypeConverter &converter,
   converter.addConversion([&](mlir::cir::LongDoubleType type) -> mlir::Type {
     return converter.convertType(type.getUnderlying());
   });
+  converter.addConversion([&](mlir::cir::FP16Type type) -> mlir::Type {
+    return mlir::FloatType::getF16(type.getContext());
+  });
   converter.addConversion([&](mlir::cir::FuncType type) -> mlir::Type {
     auto result = converter.convertType(type.getReturnType());
     llvm::SmallVector<mlir::Type> arguments;