[CIR][CIRGen][LowerToLLVM] Add address space attribute for pointer type

[seven-mile]

This is the prelude of address space support. Linked issue: #418 .

I want to solve it by these steps:

1. This PR, add the attribute and implement asm format & type conversion.
2. Enable the feature flag of address space, but break it into several other flags. And re-categorize / fix occurrences in the codebase. There is already a draft commit for this part. PR coming soon.
   • The proposed new feature flags are: addressSpaceCasting and (existing) addressSpaceInGlobalVar
3. Implement clang::CK_AddressSpaceConversion as a new kind of cir.cast. Also fix the corresponding occurrences.
4. Make ops like cir.global and cir.get_global aware of address space, and solve the latter flag.
5. Relax the restriction of default alloca address space. Then we can use correct address spaces for languages like OpenCL in future.

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Revert to this PR itself, which includes:

• custom assembly format cir.ptr<T[, $addrspace]> cir.ptr<T[, addrspace($addrspace)]>
• two build functions with optional argument addrspace = 0 extended
  • I set skipDefaultBuilders = 1, tell me if it's not a good practice😉
• CIRGen / Lowering support for the two type converter from AST -> CIR -> LLVM pipeline
• two corresponding test cases

[jopperm]

Neat!

[bcardosolopes]

Thanks for working on this. Seems almost good, some nits, just fix them and I'll merge

[bcardosolopes]

Looks great. In the future please make sure you update the PR description to only reflect what's currently done in the PR + no need to state what changed between commits because I squash them before merging.