clang-format: permit setting the path to clang in vimrc
clang-format: permit setting the path to clang in vimrc

If g:clang_format_path is set in the vimrc, that path will take precedence over
the hard coded path (which is reliant on the PATH environment variable).  This
provides an easy mechanism for switching the selected clang-format binary during
development.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@221108 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang