Merging r155984:

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r155984 | void | 2012-05-02 02:59:45 -0700 (Wed, 02 May 2012) | 4 lines

The value held in the vector may be RAUW'ed by some of the canonicalization
methods. Use a weak value handle to keep up with this.
PR12245

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_31@155985 91177308-0d34-0410-b5e6-96231b3b80d8