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.

llvm-svn: 186464