Make the contents of encoded sections SmallVector<char, N> instead of

SmallString. This makes it possible to use the length-erased SmallVectorImpl
in the interface without imposing buffer size. Thus, the size of MCInstFragment
is back down since a preallocated 8-byte contents buffer is enough.

It would be generally a good idea to rid all the fragments of SmallString as
contents, because a vector just makes more sense.

llvm-svn: 169644