[CIR][CodeGen] Fix array initialization in CIRGenExprAgg

[bruteforceboy]

Mistakenly closed #850

#850 (review)

This PR fixes array initialization for expression arguments.

Consider the following code snippet test.c:

typedef struct {
  int a;
  int b[2];
} A;

int bar() {
  return 42;
}

void foo() {
  A a = {bar(), {}};
}

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

~/clangir/clang/lib/CIR/CodeGen/CIRGenExprAgg.cpp:483: void {anonymous}::AggExprEmitter::buildArrayInit(cir::Address, mlir::cir::ArrayType, clang::QualType, clang::Expr*, llvm::ArrayRef<clang::Expr*>, clang::Expr*): Assertion `NumInitElements != 0' failed.

The error can be traced back to CIRGenExprAgg.cpp, and the fix is simple. It is possible to have an empty array initialization as an expression argument!

[bcardosolopes]

It happens. This is missing the address of the review comments from the closed PR though.

[bruteforceboy]

It happens. This is missing the address of the review comments from the closed PR though.

I am not sure if I got that, but I added a link to the review from the last PR.

[bcardosolopes]

You need to use regexes for SSA values and struct definitions, see examples in clang/test/CIR/CodeGen/abstract-cond.c and eslewhere.