[itanium demangler] Add llvm::itaniumFindTypesInMangledName()

This function calls a callback whenever a <type> is parsed.

This is necessary to implement FindAlternateFunctionManglings in LLDB, which
uses a similar hack in FastDemangle. Once that function has been updated to use
this version, FastDemangle can finally be removed.

Differential revision: https://reviews.llvm.org/D50586

llvm-svn: 339580