Fix memory leak in SubsPrimitiveParmItanium

Summary:
FastDemangle gives us a C-string that we own (which is allocated in SymbolDemangler::GetDemangledCopy).
As we are not deleting the string, we leak memory whenever we call SubsPrimitiveParmItanium.

Reviewers: javed.absar

Subscribers: kristof.beyls, chrib, lldb-commits

Differential Revision: https://reviews.llvm.org/D47418

llvm-svn: 333353