Revert "Have JIT execution take ownership of the LLVMContextRef"

This reverts commit 5c5095d25e3652c434c8d4ec178e6844877e3c2d.

Conflicts:
	src/librusti/rusti.rc
Revert "Remove all usage of the global LLVMContextRef"

This reverts commit 779191cd4b8719e8efdf69fb6da93e2a8905ca1d.

Conflicts:
	src/librustc/middle/trans/base.rs
	src/librustc/middle/trans/common.rs