[OpenCL] Allow use of double type without extension pragma.

Simply use of extensions by allowing the use of supported
double types without the pragma. Since earlier standards
instructed that the pragma is used explicitly a new warning
is introduced in pedantic mode to indicate that use of
type without extension pragma enable can be non-portable.

This patch does not break backward compatibility since the
extension pragma is still supported and it makes the behavior
of the compiler less strict by accepting code without extra
pragma statements.

Differential Revision: https://reviews.llvm.org/D100980