Revert "[analyzer] Change PathPieces to be a wrapper around an ilist of (through indirection) PathDiagnosticPieces."

Jordan rightly pointed out that we can do the same with std::list.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180746 91177308-0d34-0410-b5e6-96231b3b80d8
Just use std::list<> for PathPieces instead of complicated use of ilist.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180747 91177308-0d34-0410-b5e6-96231b3b80d8