[CodeGen] Fix clang crash on aggregate initialization of array of labels

Summary: Fix PR43700

The ConstantEmitter in AggExprEmitter::EmitArrayInit was initialized
with the CodeGenFunction set to null, which caused the crash.
Also simplify another call, and make the CGF member a const pointer
since it is public but only assigned in the constructor.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70302