Fix representation of compound literals for C++ objects with destructors.

Previously, this compound literal expression (a GNU extension in C++):

  (AggregateWithDtor){1, 2}

resulted in this AST:

 `-CXXBindTemporaryExpr [...] 'struct Point' (CXXTemporary [...])
   `-CompoundLiteralExpr [...] 'struct AggregateWithDtor'
     `-CXXBindTemporaryExpr [...] 'struct AggregateWithDtor' (CXXTemporary [...])
       `-InitListExpr [...] 'struct AggregateWithDtor'
         |-IntegerLiteral [...] 'int' 1
         `-IntegerLiteral [...] 'int' 2

Note the two CXXBindTemporaryExprs. The InitListExpr is really part of the
CompoundLiteralExpr, not an object in its own right. By introducing a new
entity initialization kind in Sema specifically for compound literals, we
avoid the treatment of the inner InitListExpr as a temporary.

 `-CXXBindTemporaryExpr [...] 'struct Point' (CXXTemporary [...])
   `-CompoundLiteralExpr [...] 'struct AggregateWithDtor'
     `-InitListExpr [...] 'struct AggregateWithDtor'
       |-IntegerLiteral [...] 'int' 1
       `-IntegerLiteral [...] 'int' 2

llvm-svn: 181212