This is a commonly useful feature to have, and we have implemented it
multiple times with different kinds of bugs. This implementation
centralizes the idea in a set of functions that we can then use from the various
tools.

Reverts r262234, which is a revert of r262232, and puts the functions
into FOrmat.h, as they are closely coupled to clang-format, and we
otherwise introduce a cyclic dependency between libFormat and
libTooling.

Patch by Eric Liu.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262323 91177308-0d34-0410-b5e6-96231b3b80d8