Fix assert instantiating string init of static variable

... when the variable's type is a typedef of a ConstantArrayType. Just
look through the typedef (and any other sugar).  We only use the
constant array type here to get the element count.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@227115 91177308-0d34-0410-b5e6-96231b3b80d8