Before:

      help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
         |
      LL |     for x in DroppingSlice(&*v).iter(); {
         |                                       +

After:

      help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
         |
      LL |     };
         |      +

This seems like a reasonable fix: since the desugared "expr_drop_temps_mut"
contains the entire desugared loop construct, its span should contain the
entire loop construct as well.

Previously, we would set up the source lines for `match` expressions so
that the code generated to perform the test of the scrutinee was matched
to the line of the arm that required the test and then jump from the arm
block to the "next" block was matched to all of the lines in the `match`
expression.

While that makes sense, it has the side effect of causing strange
stepping behavior in debuggers.

I've changed the source information so that all of the generated tests
are sourced to `match {scrutinee}` and the jumps are sourced to the last
line of the block they are inside. This resolves the weird stepping
behavior in all debuggers and resolves some instances of "ambiguous
symbol" errors in WinDbg preventing the user from setting breakpoints at
`match` expressions.

The storage markers constitute a substantial portion of all MIR
statements. At the same time, for builds without any optimizations,
the storage markers have no further use during and after MIR
optimization phase.

If storage markers are not necessary for code generation, remove them.