Change the ClangASTMap implementation to use a thread-safe wrapper over llvm::DenseMap. This helps avoid a certain class of spins per <rdar://problem/18160764>

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217888 91177308-0d34-0410-b5e6-96231b3b80d8
Add a convenience function to FormatManager to setup an empty filter (one that suppresses all children, that is)

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217891 91177308-0d34-0410-b5e6-96231b3b80d8