[LLDB][NativePDB] Fix PdbAstBuilder::GetParentDeclContext when ICF happens.

Removed `GetParentDeclContextForSymbol` as this is not necesssary. We can get
the demangled names from CVSymbol and then using it to create tag decl or
namespace decl. This also fixed a bug when icf applied.

Differential Revision: https://reviews.llvm.org/D133243