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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281286 91177308-0d34-0410-b5e6-96231b3b80d8