The vk::constant_id attribute is used to indicate that a global const
variable
represents a specialization constant in SPIR-V. This PR adds this
attribute to clang.

The documentation for the attribute is
[here](https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#specialization-constants).

The strategy is to to modify the initializer to get the value of a
specialize constant for a builtin defined in the SPIR-V backend.

Implements https://github.com/llvm/wg-hlsl/pull/287

Fixes https://github.com/llvm/llvm-project/issues/142448

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Co-authored-by: Nathan Gauër <github@keenuts.net>

To allow both to be generated.

This will get un-XFAILed but requires `-glldb` in an upcoming patch.

The code checking whether a readonly call is safe to hoist is
currently limited to only argmemonly calls. However, the actual
implementation does not depend on this in any way. It either
does an MSSA clobber walk on the memory access (which will take
all locations accessed by the call into account), or it will
look at all MemoryDefs in an entirely location-independent manner.

The current restriction dates back to the time when LICM still
supported AST, in which case this code *did* reason about the
individual pointer arguments.