Fix a couple NLL TLS spans
Some NLL TLS tests show incorrect spans for the end of function. It seems that the `TerminatorKind::Return` source info span can sometimes point at the single character after the end of the function.
Completely changing the span where the terminator is built also changes a bunch of diagnostics: small functions have more code shown unrelated to the errors at hand, wrapping symbols appear and weird-looking arrows point to the end of function, etc. So it seems this is somehow unexpectedly relied upon in making diagnostics look better and their heuristics.
So I just changed it where it matters for these few tests: the diagnostics specialized to conflict errors with thread locals.
r? `@matthewjasper`
46b18a91ab