Make Expr::isConstantInitializer match IRGen.

Sema needs to be able to accurately determine what will be
emitted as a constant initializer and what will not, so
we get accurate errors in C and accurate -Wglobal-constructors
warnings in C++.  This makes Expr::isConstantInitializer match
CGExprConstant as closely as possible.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186464 91177308-0d34-0410-b5e6-96231b3b80d8