Replace StringPiece with std::string_view in HashSet.

Update HashSet comment and related code. Also remove dead
CompilerOptions declarations with StringPiece parameters.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 123750182
Change-Id: I4a39d0e8e0392a414129369c5b304ec4813e9ed5
7 files changed