Remove all usage of the global LLVMContextRef

This allows parallel usage of the rustc library
Have JIT execution take ownership of the LLVMContextRef

Also stop leaking the ExecutionEngine created for jit code by forcibly disposing
of it after the JIT code has finished executing
auto merge of #7040 : alexcrichton/rust/issue-6511-threadsafe-llvm, r=graydon

This is a reopening of #6570, and almost fixes #6511.

Note that this doesn't actually enable building a threadsafe LLVM, because that will require an LLVM rebuild which will be bundled with the upgrades in #6713.

What this does do, however, is removes all thread-unsafe usage of LLVM from the compiler.