stage2: more progress towards mutable local variables

* implement sema for runtime deref, store pointer, coerce_to_ptr_elem,
   and store
 * identifiers support being lvalues, except for decls is still TODO
 * codegen supports load, store, ref, alloc
 * introduce more MCValue union tags to support pointers
 * add load, ref, store typed IR instructions
 * add Type.isVolatilePtr
5ccee4c986Andrew Kelley committed on 7/29/2020, 9:29:36 AMparent11d38a7
7 files changedLine totals unavailable