Simplify implementation of Rust intrinsics by using type parameters in the cache
The current implementation of intrinsics have a lot of duplication to handle different overloads of overloaded LLVM intrinsic. This PR uses the **base name and the type parameters** in the cache instead of the full, overloaded name. This has the benefit that `call_intrinsic` doesn't need to provide the full name, rather the type parameters (which is most of the time more available). This uses `LLVMIntrinsicCopyOverloadedName2` to get the overloaded name from the base name and the type parameters, and only uses it to declare the function.
(originally was part of rust-lang/rust#140763, split off later)
`@rustbot` label A-codegen A-LLVM
r? codegen
cc87afd8c0