[analyzer] pr34766: Fix a crash on explicit std::initializer_list constructor.

We didn't support the following syntax:

  (std::initializer_list<int>){12}

which suddenly produces CompoundLiteralExpr that contains
CXXStdInitializerListExpr.

Lift the assertion and instead pass the value through CompoundLiteralExpr
transparently, as it doesn't add much.

Differential Revision: https://reviews.llvm.org/D39803

llvm-svn: 319058