Function prelude associates with caller's source position

[jdm]

With a simple setup like

pub fn f() {
  io::println("foo");
}

pub fn main() {
  f();
}

breaking in main and stepping into f yields odd behaviour in gdb. Specifically, the first step into f shows pub fn main() {, the second shows f();, and the third takes us to the proper io::println("foo");. Disassembling at each step shows the first step takes us inside f into its prelude, so obviously we're not updating the source position correctly when generating the function.

[jdm]

Tagging @michaelwoerister so he sees this.

[jdm]

Can't reproduce this any more with master. Shrug.