llvm: make debuggers actually usable

`@llvm.dbg.value` is absolutely useless, adding a temporary alloca
to store the constant in will make it actually show up in debuggers.
The effect on performance should be minimal since there is only one
store and it the change is not applied to ReleaseSafe builds.

```zig
fn foo(a: u32, b: []const u8, c: bool, d: enum { yes, no }) void {
    _ = a; _ = b; _ = c; _ = d;
}
```
before:
```
Breakpoint 1, a.foo (a=<optimized out>, b=..., c=<optimized out>, d=<optimized out>) at a.zig:18
18          _ = d;
```
after:
```
Breakpoint 1, a.foo (a=1, b=..., c=false, d=yes) at a.zig:15
15          _ = a; _ = b; _ = c; _ = d;
(gdb) p b
$1 = {ptr = 0x20854f <a.main.anon_3888> "bar", len = 3}
```

0 files changed, 0 insertions, 0 deletions