[clang-format] Switch to case-insensitive header matching and use it to

improve support for LLVM-style include sorting.

This really is a collection of improvements to the rules for LLVM
include sorting:
- We have gmock headers now, so it adds support for those to one of the
  categories.
- LLVM does use 'FooTest.cpp' files to test 'Foo.h' so it adds that
  suffix for finding a main header.
- At times the test file's case may not match the header file's case, so
  switch to case-insensitive regex matching of header names.

With this set of changes, I can't spot any misbehaviors when re-sorting
all of LLVM's unittest '#include' lines.

Thanks to Eric and Daniel for help testing and refining the patch during
review!

Differential Revision: https://reviews.llvm.org/D33932

llvm-svn: 306759