clang-format: Use pipes instead of temporary files for most lit tests.

This makes the format tests look more like most other FileCheck tests in clang.

The multiple-inputs tests still use temp files, to make sure that the file
input code in clang-format stays tested.

Stop stripping out the comment lines in style-on-command-line.cpp as they don't
get in the way and it makes the test simpler. Also remove 2>&1s on the tests in
that file that don't need it.

http://reviews.llvm.org/D13852


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250706 91177308-0d34-0410-b5e6-96231b3b80d8