[analyzer] C++ initializers may require cleanups; look through these.

When the analyzer sees an initializer, it checks if the initializer
contains a CXXConstructExpr. If so, it trusts that the CXXConstructExpr
does the necessary work to initialize the object, and performs no further
initialization.

This patch looks through any implicit wrapping expressions like
ExprWithCleanups to find the CXXConstructExpr inside.

Fixes PR15070.