[DebugInfo] Don't dereference a dyn_cast<PDBSymbolData> result. NFCI.

The static analyzer is warning about a potential null dereference - but as we're in DataMemberLayoutItem we should be able to guarantee that the Symbol is a PDBSymbolData type, allowing us to use cast<PDBSymbolData> - and if not assert will fire for us.

llvm-svn: 371933