[ARM,MVE] Add intrinsics for 'administrative' vector operations.

This batch of intrinsics includes lots of things that move vector data
around or change its type without really affecting its value very
much. It includes the `vreinterpretq` family (cast one vector type to
another); `vuninitializedq` (create a vector of a given type with
don't-care contents); and `vcreateq` (make a 128-bit vector out of two
`uint64_t` halves).

These are all implemented using completely standard IR that's already
tested in existing LLVM unit tests, so I've just written a clang test
to check the IR is correct, and left it at that.

I've also added some richer infrastructure to the MveEmitter Tablegen
backend, to make it specify the exact integer type of integer
arguments passed to IR construction functions, and wrap those
arguments in a `static_cast` in the autogenerated C++. That was
necessary to prevent an overloading ambiguity when passing the integer
literal `0` to `IRBuilder::CreateInsertElement`, because otherwise, it
could mean either a null pointer `llvm::Value *` or a zero `uint64_t`.

Reviewers: ostannard, MarkMurrayARM, dmgreen

Subscribers: kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70133

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