This implements RFC 80. See apache/tvm-rfcs#83.

Summary of changes:
- Created an `LLVMScope` class. Uses of LLVM functions and data struc-
tures should be contained within the lifetime of an object of this class.
LLVMScope object contains LLVMContext, and implements member functions
to deserialize an llvm::Module.
- Created an `LLVMTarget` class. Once an LLVMScope object has been
created, an object of LLVMTarget class can be created from TVM target
string, or Target object for "llvm" target. Once LLVM command line flags
are added to the "llvm" target, one of the goals of this object will be
to save/restore relevant LLVM global state. Another objective for the
LLVMTarget object is to be a single location for all LLVM-related
compilation structures and options (such as TargetMachine, FastMathFlags,
etc.)

Move empty implementations of EnterWithScope/ExitWithScope to header
since it helps see that these functions are only stubs.