PR22465: when performing list-initialization for a class type C, if we see an

PR22465: when performing list-initialization for a class type C, if we see an
initializer of the form {x}, where x is of type C or a type derived from C,
perform *non-list* initialization of the entity from x, but create a
CXXConstructExpr that knows that we used list-initialization syntax.

Plus some fixes to ensure we mangle correctly in this and related cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228276 91177308-0d34-0410-b5e6-96231b3b80d8