Allow transformation of VariableArray to ConstantArray.

In the following code:

    struct A { static const int sz; };
    template<class T> void f() { T arr[A::sz]; }

the array 'arr' is represented as a variable size array in the template.
If 'A::sz' gets value below in the translation unit, the array in
instantiation can turn into constant size array.

This change fixes PR18633.

Differential Revision: http://llvm-reviews.chandlerc.com/D2688


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200899 91177308-0d34-0410-b5e6-96231b3b80d8