Replacing an empty switch with its moral equivalent.  No functional changes intended.
[Option] Store arg strings in a set backed by a BumpPtrAllocator

No functionality change.

This is preparing to move response file parsing into lib/Option so it
can be shared between clang and lld.  This change isn't just a
micro-optimization.  Clang's driver uses a std::set<std::string> to
unique arguments while parsing response files, so this matches that.
Improve formatting of operator<< chains.

Before:
  llvm::outs() << "aaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaaaaa << "aaaaaaaaaaaaaaaa: "
               << aaaaaaaaaaaaaaaa << "aaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaaaaa;

After:
  llvm::outs() << "aaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaaaaa
               << "aaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaaaaa
               << "aaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaaaaa;
Test commit to see if write access works.
Improvement of change r186320.

Fixed a test that by now passed for the wrong reason.

Before:
  llvm::outs() << "aaaaaaaaaaaaaaaaaaa: " << aaaaaaaaaaaaa(
                                                 aaaaaaaaaaaaaaaaaaaaaaaaaaaa);
After:
  llvm::outs() << "aaaaaaaaaaaaaaaaaaa: "
               << aaaaaaaaaaaaa(aaaaaaaaaaaaaaaaaaaaaaaaaaaa);

Also reformatted Format.cpp with the latest changes (1 formatting fix
and 1 layout change of a <<-chain).