Allocate CodeCompletionString and all of its components in a

BumpPtrAllocator, rather than manually new/delete'ing them. This
optimization also allows us to avoid allocating memory for and copying
constant strings (e.g., "return", "class").

This also required embedding the priority and availability of results
within the code completion string, to avoid extra memory allocation
within libclang.

llvm-svn: 124673