[JITLink][ORC] Add LinkGraph::allocateCString method.

Renames the existing allocateString method to allocateContent and adds a pair of
allocateCString methods.

The previous allocateString method did not include a null-terminator. It behaved
the same as allocateContent except with a Twine input, rather than an
ArrayRef<char>. Renaming allocateString to allocateBuffer (overloading the
existing method) makes this clearer.

The new allocateCString methods allocate the given content plus a
null-terminator character, and return a buffer covering both the string and
null-terminator. This makes them suitable for creating c-string content for
jitlink::Blocks.

Existing users of the old allocateString method have been updated to use the
new allocateContent overload.