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

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.

llvm-svn: 281286

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