MIR-inlined functions called with constants should put the constant in debug info

[scottmcm]

The following code calls is_digit with a constant radix:

rust/tests/mir-opt/issues/issue_59352.rs

Lines 13 to 16 in 37b22cf

	pub fn num_to_digit(num: char) -> u32 {
	// CHECK-NOT: panic
	if num.is_digit(8) { num.to_digit(8).unwrap() } else { 0 }
	}

The MIR inliner preserves a local variable that's "holding" the value

rust/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.mir

Lines 7 to 10 in 37b22cf

	let mut _3: u32; // in scope 0 at $DIR/issue_59352.rs:+2:12: +2:23
	scope 1 (inlined char::methods::<impl char>::is_digit) { // at $DIR/issue_59352.rs:15:12: 15:23
	debug self => _1; // in scope 1 at $SRC_DIR/core/src/char/methods.rs:LL:COL
	debug radix => _3; // in scope 1 at $SRC_DIR/core/src/char/methods.rs:LL:COL

But it's actually dead -- nothing sets _3 to anything, so that debug info node is useless.

(I'm working on a PR that will remove always-uninitialized debug info like that _3, which is how I noticed this. So I opened this in case the debuginfo folks want this to work instead of just being dropped.)

[cjgillot]

ConstDebugInfo does the general work of uplifting to debuginfo. I have #107404 trying to turn it by default.