Restructure checking for, and warning on, lifetime extension.

Restructure checking for, and warning on, lifetime extension.

This change implements C++ DR1696, which makes initialization of a
reference member of a class from a temporary object ill-formed. The
standard wording here is imprecise, but we interpret it as meaning that
any time a mem-initializer would result in lifetime extension, the
program is ill-formed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@337226 91177308-0d34-0410-b5e6-96231b3b80d8