Target features getting erased when using `#[export_name]` with names of LLVM intrinsics

TIL Rust "supports" #[export_name] with names of LLVM intrinsics! The following code compiles no problem

#[unsafe(export_name = "llvm.x86.rdtsc")]
fn foo() {
    // ...
}

(FYI llvm.x86.rdtsc is a very real LLVM intrinsic, used to read the timestamp counter value in x86)
which imo should be hard error anyway. But then if we combine this monstrosity with #[target_feature] (or -C target-feature for that matter), all the target feature data for that function is erased. This is possible even in stable Rust, both in debug and release profile.

Godbolt link https://godbolt.org/z/h7jdrsY54

Meta

Doing a naive bisect on Godbolt, I found out that this bug was introduced in 1.29.0!!
https://godbolt.org/z/8M9a4vs9Y

The solution is probably just disallow any #[export_name]'s that try to masquerade as LLVM intrinsics (i.e. name starts with llvm.), which will mean reverting to pre-1.29.0 behavior.

@rustbot label T-compiler

Reproduce the provided Rust snippet and compare the Godbolt results for current Rust and the 1.29.0 regression link, including #[export_name] with #[target_feature]. Then trace the compiler handling of exported names and target features; done means LLVM-intrinsic-looking export names are rejected and target feature data is no longer erased.