[clang-format][NFC] Code Tidies in UnwrappedLineFormatter

* Give I[1] and I[-1] a name:
  - Easier to understand
  - Easier to debug (since you don't go through operator[] everytime)
* TheLine->First != TheLine->Last follows since last is a l brace and
  first isn't.
* Factor the check for is(tok::l_brace) out.
* Drop else after return.

Differential Revision: https://reviews.llvm.org/D115060