[CIR][CIRGen] Add dynamic builtin alloca intrinsics support

[orbiri]

This patch adds the CIRGen for the following builtin functions:

• alloca;
• _alloca;
• __builtin_alloca;
• __builtin_alloca_uninitialized.

Missing support to add in the future:

• Non-default auto initialization setting. The default is to not initialize the allocated buffer, which is simpler to implement. This commit is leaving the skeleton to implement this feature following clang's codegen pattern.
• It may be possible that the frontend has set non-default address space for the alloca's return value. This is the case for OpenCL or AMDGPU codes for example. This is handled in clang codegen via address space cast, and is left for future implementation. This commit introduces a guard-rail around this behaviour.

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P.S.: I'm aware this is rather an "esoteric" feature to be supported and I'm sure there are more valuable features to implement before that! Implementing this feature was my method of learning more about the development flow for CIR - working along side clang's codegen, testing and debugging flow, etc.

[orbiri]

Embarrassing formatting and lit-tests errors fixed and verified locally 🤦

[orbiri]

Thanks for the review!! Will address the comments in the coming days. New push is just rebasing on top of Nathan's rebase, didn't address any comment yet! :)

[orbiri]

Addressed all comments, main changes:

• Addressed ASTAllocaAddressSpace support in a separate PR (#551 )
• Added createAlloca builder helpers in CIRBaseBuilder.h, and replaced usages with it.

[orbiri]

@bcardosolopes do you think we can land this? :)