Audit uses of Sema.requireRuntimeBlock

[mlugg]

Sema uses the requireRuntimeBlock function to emit a compile error when performing a runtime operation in a comptime block. However, most uses of this function are actually incorrect. The consensus Andrew and I have reached, to make sure expressions like comptime runtime_array.len work, is that non-side-effecting expressions are allowed to be runtime-known in a comptime scope. It's never possible for a runtime value to be "leaked" out of a comptime block, nor to be meaningfully used within one, so this never allows a comptime scope to actually emit real runtime code (Sema discards instructions of the Block being comptime-evaluated).

All uses of requireRuntimeBlock need to be audited, and the majority simply removed. The only ones remaining should be those which have side effects, such as storing to a runtime pointer.