[CFG] [analyzer] Add construction context to C++ return-by-value call elements.

This patch adds a new CFGStmt sub-class, CFGCXXRecordTypedCall, which replaces
the regular CFGStmt for the respective CallExpr whenever the CFG has additional
information to provide regarding the lifetime of the returned value.

This additional call site information is represented by a ConstructionContext
(which was previously used for CFGConstructor elements) that provides references
to CXXBindTemporaryExpr and MaterializeTemporaryExpr that surround the call.

This corresponds to the common C++ calling convention solution of providing
the target address for constructing the return value as an auxiliary implicit
argument during function call.

One of the use cases for such extra context at the call site would be to perform
any sort of inter-procedural analysis over the CFG that involves functions
returning objects by value. In this case the elidable constructor at the return
site would construct the object explained by the context at the call site, and
its lifetime would also be managed by the caller, not the callee.

The extra context would also be useful for properly handling the return-value
temporary at the call site, even if the callee is not being analyzed
inter-procedurally.

Differential Revision: https://reviews.llvm.org/D44120

llvm-svn: 327343