stage2: more principled approach to comptime references

 * AIR no longer has a `variables` array. Instead of the `varptr`
   instruction, Sema emits a constant with a `decl_ref`.
 * AIR no longer has a `ref` instruction. There is no longer any
   instruction that takes a value and returns a pointer to it. If this
   is desired, Sema must either create an anynomous Decl and return a
   constant `decl_ref`, or in the case of a runtime value, emit an
   `alloc` instruction, `store` the value to it, and then return the
   `alloc`.
 * The `ref_val` Value Tag is eliminated. `decl_ref` should be used
   instead. Also added is `eu_payload_ptr` which points to the payload
   of an error union, given an error union pointer.

In general, Sema should avoid calling `analyzeRef` if it can be helped.
For example in the case of field_val and elem_val, there should never be
a reason to create a temporary (alloc or decl). Recent previous commits
made progress along that front.

There is a new abstraction in Sema, which looks like this:

    var anon_decl = try block.startAnonDecl();
    defer anon_decl.deinit();
    // here 'anon_decl.arena()` may be used
    const decl = try anon_decl.finish(ty, val);
    // decl is typically now used with `decl_ref`.

This pattern is used to upgrade `ref_val` usages to `decl_ref` usages.

Additional improvements:

 * Sema: fix source location resolution for calling convention
   expression.
 * Sema: properly report "unable to resolve comptime value" for loads of
   global variables. There is now a set of functions which can be
   called if the callee wants to obtain the Value even if the tag is
   `variable` (indicating comptime-known address but runtime-known value).
 * Sema: `coerce` resolves builtin types before checking equality.
 * Sema: fix `u1_type` missing from `addType`, making this type have a
   slightly more efficient representation in AIR.
 * LLVM backend: fix `genTypedValue` for tags `decl_ref` and `variable`
   to properly do an LLVMConstBitCast.
 * Remove unused parameter from `Value.toEnum`.

After this commit, some test cases are no longer passing. This is due to
the more principled approach to comptime references causing more
anonymous decls to get sent to the linker for codegen. However, in all
these cases the decls are not actually referenced by the runtime machine
code. A future commit in this branch will implement garbage collection
of decls so that unused decls do not get sent to the linker for codegen.
This will make the tests go back to passing.

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