Remove unused "append" parameter from FindTypes API

I noticed that SymbolFileDWARFDebugMap::FindTypes was implementing it
incorrectly (passing append=false in a for-loop to recursive calls to
FindTypes would yield only the very last set of results), but instead
of fixing it, removing it seemed like an even better option.

rdar://problem/54412692

Differential Revision: https://reviews.llvm.org/D68171

llvm-svn: 373224