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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319058 91177308-0d34-0410-b5e6-96231b3b80d8