Sema: implement comptime variables

Sema now properly handles alloc_inferred and alloc_inferred_mut ZIR
instructions inside a comptime execution context. In this case it
creates Decl objects and points to them with the new `decl_ref_mut`
Value Tag. `storePtr` is updated to mutate such Decl types and values.
In this case it destroys the old arena and makes a new one, preventing
memory growth during comptime code execution.

Additionally:

 * Fix `storePtr` to emit a compile error for a pointer comptime-known
   to be undefined.
 * Fix `storePtr` to emit runtime instructions for all the cases that a
   pointer is comptime-known but does not support comptime
   dereferencing, such as `@intToPtr` on a hard-coded address, or an
   extern function.
 * Fix `ret_coerce` not coercing inside inline function call context.

test/behavior/basic.zig

@@ -0,0 +1,9 @@
+// normal comment
+
+/// this is a documentation comment
+/// doc comment line 2
+fn emptyFunctionWithComments() void {}
+
+test "empty function with comments" {
+    emptyFunctionWithComments();
+}