Merging r281286:

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r281286 | rtrieu | 2016-09-12 18:20:40 -0700 (Mon, 12 Sep 2016) | 6 lines

Fix interaction between serialization and c++1z feature.

In c++1z, static_assert is not required to have a StringLiteral message, where
previously it was required.  Update the AST Reader to be able to handle a
null StringLiteral.

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_39@286667 91177308-0d34-0410-b5e6-96231b3b80d8