Improve checking for dynamic initializers of dllimport fields in template instantiation

We would previously assert if the initializer was dependent. I also think that
checking isConstantInitializer is more correct here than checkInitIsICE.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210505 91177308-0d34-0410-b5e6-96231b3b80d8