[CFG] [analyzer] pr41142: C++17: Skip transparent InitListExprs in ExprEngine.

r356634 didn't fix all the problems caused by r356222 - even though simple
constructors involving transparent init-list expressions are now evaluated
precisely, many more complicated constructors aren't, for other reasons.

The attached test case is an example of a constructor that will never be
evaluated precisely - simply because there isn't a constructor there (instead,
the program invokes run-time undefined behavior by returning without a return
statement that should have constructed the return value).

Fix another part of the problem for such situations: evaluate transparent
init-list expressions transparently, so that to avoid creating ill-formed
"transparent" nonloc::CompoundVals.

Differential Revision: https://reviews.llvm.org/D59622

llvm-svn: 356969