Use 'critical' reduction method when 'atomic' is not available but requested.

In case atomic reduction method is not available (the compiler can't generate
it) the assertion failure occurred if KMP_FORCE_REDUCTION=atomic was specified.
This change replaces the assertion with a warning and sets the reduction method
to the default one - 'critical'.

Patch by Olga Malysheva

Differential Revision: https://reviews.llvm.org/D23990

llvm-svn: 280519