ICE for adding block to str

[klutzy]

fn main() {
    let a = "a";
    a += { "b" }; // oops
}

This causes ICE as well as normal error:

a.rs:3:4: 3:16 error: binary operation + cannot be applied to type `&'static str`
a.rs:3     a += { "b" }; // oops
           ^~~~~~~~~~~~
error: internal compiler error: no type for node 16: block { "b" } (id=16) in fcx 0x7fdaac1165b0

As a consequence, a += format!(...); triggers same error. Same for a += {};.

cc #9942 and #10091 since they just share same ICE message.

[alexcrichton]

This has been fixed, flagging as needstest.