[mlir][llvm] Add isVarArg flag to lookupOrCreateFn.

The function is a helper for looking up a function by name or creating
it if it doesn't exist. The arguments allow to specify the signature of
the function, if it needs to be created, but do not expose the varArg
parameter of LLVMFunctionType. This patch exposes adds an optional
parameter with a default value of false such that existing usage
continue to work as before.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D144256