[CFG] Provide construction contexts for temporaries in conditional operators.

When a lifetime-extended temporary is on a branch of a conditional operator,
materialization of such temporary occurs after the condition is resolved.

This change allows us to understand, by including the MaterializeTemporaryExpr
in the construction context, the target for temporary materialization in such
cases.

Differential Revision: https://reviews.llvm.org/D43483

llvm-svn: 326019