[lldb] Filter duplicates in Target::GetScratchTypeSystems

`Target::GetScratchTypeSystems` returns the list of scratch TypeSystems. The
current implementation is iterating over all LanguageType values and retrieves
the respective TypeSystem for each LanguageType.

All C/C++/Obj-C LanguageTypes are however mapped to the same
ScratchTypeSystemClang instance, so the current implementation adds this single
TypeSystem instance several times to the list of TypeSystems (once for every
LanguageType that we support).

The only observable effect of this is that `SBTarget.FindTypes` for builtin
types currently queries the ScratchTypeSystemClang several times (and also adds
the same result several times).

Reviewed By: bulbazord, labath

Differential Revision: https://reviews.llvm.org/D111931