Rust enums with discriminators greater than u64::MAX are not encoded properly in debuginfo

[wesleywiser]

Rust enums with discriminators greater than u64::MAX are not encoded properly because LLVM lacks the appropriate APIs to do so. As a result, we assert!() in some situations:

rust/compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs

Lines 416 to 419 in 8006510

	// NOTE(eddyb) do *NOT* remove this assert, until
	// we pass the full 128-bit value to LLVM, otherwise
	// truncation will be silent and remain undetected.
	assert_eq!(value as u64 as u128, value);

#![feature(repr128)]

#[repr(u128)]
pub enum Foo {
    Lo,
    Hi = 1 << 64,
    Bar = 18_446_745_000_000_000_123,
}

pub fn foo() -> Option<Foo> {
    None
}

To fix this, we need APIs in LLVM to pass the correct value and have it be encoded in the DWARF.

Split off of #59509