split CrateContext into shared and local pieces

split CrateContext into shared and local pieces

Break up `CrateContext` into `SharedCrateContext` and `LocalCrateContext`.  The
local piece corresponds to a single compilation unit, and contains all
LLVM-related components.  (LLVM data structures are tied to a specific
`LLVMContext`, and we will need separate `LLVMContext`s to safely run
multithreaded optimization.)  The shared piece contains data structures that
need to be shared across all compilation units, such as the `ty::ctxt` and some
tables related to crate metadata.