[CIR][Lowering] Fix lowering for multi dimensional array

[bruteforceboy]

This PR fixes the lowering for multi dimensional arrays.

Consider the following code snippet test.c:

void foo() {
  char arr[4][1] = {"a", "b", "c", "d"};
}

When ran with bin/clang test.c -Xclang -fclangir -Xclang -emit-llvm -S -o -, It produces the following error:

~/clangir/llvm/include/llvm/Support/Casting.h:566: decltype(auto) llvm::cast(const From&) [with To = mlir::ArrayAttr; From = mlir::Attribute]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

The bug can be traced back to LoweringHelpers.cpp. It considers the values in the array as integer types, and this causes an error in this case.

This PR updates convertToDenseElementsAttrImpl when the array contains string attributes. I have also added one more similar test. Note that in the tests I used a literal match to avoid matching as regex, so !dbg is useful.

[bcardosolopes]

LGTM with some minor changes needed.

[bruteforceboy]

I just made a small change to LoweringHelpers.cpp. I noticed in some cases there were missing trailing zeros. I have added an extra test, charInit1, to array-init.c please check it out.

[bcardosolopes]

LGTM!

While here, extra question: if one of these arrays has more elements than what's used in the written initialization, is it getting proper zeros at the end? Perhaps add more tests for that in a following up PR?