Improve error handling for SmallVector programming errors

This patch changes errors in `SmallVector::grow` that are independent of
memory capacity to be reported using report_fatal_error or
std::length_error instead of report_bad_alloc_error, which falsely signals
an OOM.

It also cleans up a few related things:
- makes report_bad_alloc_error to print the failure reason passed
  to it.
- fixes the documentation to indicate that report_bad_alloc_error
  calls `abort()` not "an assertion"
- uses a consistent name for the size/capacity argument to `grow`
  and `grow_pod`

Reviewed By: mehdi_amini, MaskRay

Differential Revision: https://reviews.llvm.org/D86892