clang-format: Extend detection of the "main" #include to use the filename

Before, the first (non-system) header in a file was considered to be
the main include. This is conservative as it makes clang-format change
the #include order less often. Instead implement some basic usage of
the filename itself. With this patch, clang-format considers every
header to be a main include if the header file's basename is a prefix
to the filename the #include is in.

llvm-svn: 256148