[CFG] [analyzer] Add construction context for implicit constructor conversions.

Implicit constructor conversions such as A a = B() are represented by
surrounding the constructor for B() with an ImplicitCastExpr of
CK_ConstructorConversion kind, similarly to how explicit constructor conversions
are surrounded by a CXXFunctionalCastExpr. Support this syntax pattern when
extracting the construction context for the implicit constructor that
performs the conversion.

Differential Revision: https://reviews.llvm.org/D44051

llvm-svn: 327096