make function calls contain debug stmt line/column info

[andrewrk]

Zig version: 0.10.0-dev.1311+5ea94e771

const std = @import("std");

pub fn main() void {
    var x: i32 = 1234;
    x += foo() +
        bar();
}

fn foo() i32 {
    return 1;
}

fn bar() i32 {
    @panic("crash");
}

Here you can see in a debugging session, the traceback points back to the wrong function call:

$ ./stage2/bin/zig build-exe test3.zig -fLLVM
$ gdb ./test3 -ex run
Program received signal SIGTRAP, Trace/breakpoint trap.
0x00000000002019e7 in builtin.default_panic (msg=..., error_return_trace=<optimized out>) at /home/andy/dev/zig/lib/std/builtin.zig:758
758	            @breakpoint();
(gdb) up
#1  0x0000000000202127 in test3.bar () at test3.zig:14
14	    @panic("crash");
(gdb) 
#2  0x0000000000201b42 in test3.main () at test3.zig:5
5	    x += foo() +
(gdb) 

This is because in the ZIR we only emit dbg_stmt for the var decl, not each function call:

      %11 = dbg_stmt(3, 5)
      %12 = load(%7) node_offset:5:7
      %13 = typeof(%12) node_offset:5:7
      %14 = decl_val("foo") token_offset:5:10
      %15 = call(.auto, %14, []) node_offset:5:13
      %16 = decl_val("bar") token_offset:6:9
      %17 = call(.auto, %16, []) node_offset:6:12
      %18 = add(%15, %17) node_offset:5:16
      %19 = add(%12, %18) node_offset:5:7
      %20 = store(%7, %19)
      %21 = ret_tok(@Ref.void_value) token_offset:7:1

The two call instructions do have a source node, but only the index is communicated; not line/column info for use by debug info.

We need to augment the call ZIR instruction with the same information as provided by dbg_stmt and then treat each function call as an implicit dbg_stmt instruction.