[OpenCL] Improve diagnostic for placement new

Without an explicit declaration for placement new, clang would reject
uses of placement new with "'default new' is not supported in OpenCL
C++".  This may mislead users into thinking that placement new is not
supported, see e.g. PR42060.

Clarify that placement new requires an explicit declaration.

Differential Revision: https://reviews.llvm.org/D63561

llvm-svn: 364423