LLVM code generation

[shoaibkamil]

Opening PR for code review purposes.

This adds LLVM code generation for taco. There are currently several caveats to the implementation, which I plan to rectify in further PRs. At the moment:

• Only LLVM 7.0 is supported
• This does not implement an LLVM JIT yet; instead, we use llc to compile generated bitcode, similar to the approach we use to compile generated C99 code
• Vectorization and parallelization are not supported
• The CMake configuration currently requires LLVM; this should be changed so that LLVM support is only built if LLVM is found, and otherwise C99 code generation is built.
• Only the new lowering machinery is supported, because the old machinery does not produce correctly-typed low-level IR

C99 codegen remains the default. In order to test LLVM codegen, TACO_TARGET should be set to x86-macos or similar, and TACO_LLC should point to your llc binary. E.g. on my machine:

TACO_TARGET=x86-macos TACO_LLC=/usr/local/cellar/llvm/7.0.0/bin/llc bin/debug/taco-test lower