Fix an unused variable warning in release builds.

``num_params`` was unused in RenderScript ABI fixup pass ``cloneToStructRetFnTy``
and was only used in an `assert()` that the number of function parameters for the cloned
function was correct.

Now we actually use this variable, rather than recomputing it, and avoid the unused variable
warning when building without asserts enabled.

Subscribers: lldb-commits
llvm-svn: 277608