[CFG] Provide construction contexts for lifetime-extended temporaries.

When constructing a temporary that is going to be lifetime-extended through a
MaterializeTemporaryExpr later, CFG elements for the respective constructor
can now be queried to obtain the reference to that MaterializeTemporaryExpr
and therefore gain information about lifetime extension.

This may produce multi-layered construction contexts when information about
both temporary destruction and lifetime extension is available.

Differential Revision: https://reviews.llvm.org/D43477

llvm-svn: 326014