This patch adds a regular expression to configure suffixes of an
included file to check whether it is the "main" include of the current
file. Previously, clang-format has allowed arbitrary suffixes on the
formatted file, which is still the case when no IncludeMainRegex is
specified.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263943 91177308-0d34-0410-b5e6-96231b3b80d8