Added support for setting the solver from llvm-lit command line
e.g.

$ llvm-lit --param=solver=z3 .
Added OutputCheck tool as a submodule for use for llvm-lit tests.
Added %OutputCheck substitution to llvm-lit tests. A llvm-lit parameter
has also been added which can pass extra flags to OutputCheck e.g.

$ llvm-lit --param=outputcheck_params='-l debug' -v .