[libclang/codecompletion] Make sure the top-level decl hash takes into account ImportDecls.

The top-level hash is used to determine if we need to update the global code-completion results.
ImportDecls did not affect the hash so a newly introduced ImportDecl would not trigger an update of the global results.

rdar://14202797

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184782 91177308-0d34-0410-b5e6-96231b3b80d8