More precise spans for temps and their drops
This PR has two main enhancements:
1. when possible during code generation for a statement (like `expr();`), pass along the span of a statement, and then attribute the drops of temporaries from that statement to the statement's end-point (which will be the semicolon if it is a statement that is terminating by a semicolon).
2. when evaluating a block expression into a MIR temp, use the span of the block's tail expression (rather than the span of whole block including its statements and curly-braces) for the span of the temp.
Each of these individually increases the precision of our diagnostic output; together they combine to make a much clearer picture about the control flow through the spans.
Fix#54382
6ca7bc0eb8