Add test where diagnostic span points to just the inlined function's body

Author: oli-obk

src/test/ui/const_prop/inline_spans.rs

@@ -0,0 +1,14 @@
+// build-fail
+// compile-flags: -Zmir-opt-level=2
+
+#![deny(warnings)]
+
+fn main() {
+    let _ = add(u8::MAX, 1);
+}
+
+#[inline(always)]
+fn add(x: u8, y: u8) -> u8 {
+    x + y
+    //~^ ERROR this arithmetic operation will overflow
+}

src/test/ui/const_prop/inline_spans.stderr

@@ -0,0 +1,10 @@
+error: this arithmetic operation will overflow
+  --> $DIR/inline_spans.rs:12:5
+   |
+LL |     x + y
+   |     ^^^^^ attempt to compute `u8::MAX + 1_u8` which would overflow
+   |
+   = note: `#[deny(arithmetic_overflow)]` on by default
+
+error: aborting due to previous error
+