Lack of support for the GCC vector extensions was causing a bunch of sanity failures in the syntax zoo. This PR adds minimal IR generation support for these types.
Added `VectorAggregateLiteral`, and factored most of `ArrayAggregateLiteral` out into the common base class `ArrayOrVectorAggregateLiteral`. I'd be happy to merge these all into `ArrayAggregateLiteral` if we don't care about the distinction.
Made a few tweaks to `TranslatedArrayExpr` to compute the element type by looking at the result type of the `ArrayExpr`, not the type of the base operand. Note that this means that for `T a[10]; a[i] = foo;`, the result of the `PointerAdd` for `a[i]` will now be `glvalue<T>`, not `T*`. This is actually more faithful to the source language, and has no semantic difference on the IR.
Added some missing `getInstructionElementSize()` overrides.
Added the new `BuiltIn` opcode, renamed the existing `BuiltInInstruction` to `BuiltInOperationInstruction`, and made any `BuiltInOperation` that we don't specifically handle translate to `BuiltIn`. `BuiltInOperationInstruction` now has a way to get the specific `BuiltInOperation`.
Added `getCanonicalQLClass()` overrides for `GNUVectorType` and `BuiltInOperation`.
Added a simple IR test for vector types.
