go/ssa/interp: delete SSA bodies from standalone functions in "reflect"

...like we do for "runtime" functions, so that they fail informatively
if executed.  They all need intrinsics, but only some are yet defined.

Also:
- added test for issue 9462
- "BUG" in test output is now a failure in all tests (not just $GOROOT tests)
- added intrinsic for reflect.SliceOf
- show dynamic type of panic value

Fixes issue 9462

Change-Id: I3a504c7faeed81e922fedc7dd59222717f3a7e95
Reviewed-on: https://go-review.googlesource.com/2145
Reviewed-by: Robert Griesemer <gri@golang.org>