Model temporary lifetime-extension explicitly in the AST. Use this model to

Model temporary lifetime-extension explicitly in the AST. Use this model to
handle temporaries which have been lifetime-extended to static storage duration
within constant expressions. This correctly handles nested lifetime extension
(through reference members of aggregates in aggregate initializers) but
non-constant-expression emission hasn't yet been updated to do the same.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183283 91177308-0d34-0410-b5e6-96231b3b80d8