Associate clang-format formatter with C and Obj-C

[dbarnett]

In clang-format's help, it calls itself "a tool to format C/C++/Obj-C code", but the current AppliesToBuffer check on the clang-format formatter only checks for cpp, proto, and javascript filetypes.

We should enable it as a default formatter for filetypes c, objc, and objcpp so that if people use :FormatCode on these filetypes with no explicit formatter, they will get code formatted with clang-format instead of seeing Not available. codefmt doesn't have a default formatter for this buffer.

[artasparks]

Is this a trivial change to the list of filetypes?

[dbarnett]

Yeah, I think I reported the feature request instead of implementing it directly because I wanted a sanity check that it gives good results from someone who uses C and Obj-C more, and also to ensure that enabling it by default won't cause headaches for googlers.