Performance: Add a set of visited SymbolFiles to the other FindFiles variant.

This is basically the same bug as in r260434.

SymbolFileDWARF::FindTypes has exponential worst-case when digging
through dependency DAG of .pcm files because each object file and .pcm
file may depend on an already-visited .pcm file, which may again have
dependencies. Fixed here by carrying a set of already visited
SymbolFiles around.

rdar://problem/56993424

Differential Revision: https://reviews.llvm.org/D70106