rustc with LLVM 5 reports invalid DIExpression

[cuviper]

This is a known problem from #43370, but I feel it deserves its own issue. When using rustc compiled with LLVM 5.0.1, compiling with debuginfo often prints "invalid expression" messages to stderr, and AFAICT affected functions don't get any debuginfo written at all. For example:

$ cat foo.rs
fn main() {
    (0..10).for_each(|i| println!("{}", i));
}

$ rustc -g foo.rs
invalid expression
!40 = !DIExpression(6, 34, 0)

Previous analysis in #43370 tied it to this declare_local call, and @eddyb offered a possible solution in #43370 (comment).

[cuviper]

LLVM <= 4.0 used a non-standard interpretation of DW_OP_plus (the 34 in that DIExpression). In the DWARF standard, this adds two items on the expressions stack. LLVM's behavior was more like DWARF's DW_OP_plus_uconst -- adding a constant that follows the op -- and the patch series starting with D33892 switched to the standard DWARF interpretation.

I'll work out the compatibility macros and send a PR soon.