Make CastExpr::getSubExprAsWritten look through implicit temporary under CK_ConstructorConversion

With

    struct S1 {};
    struct S2 { operator S1(); };
    S1 f(S2 s) { return static_cast<S1>(s); }

the static_cast expr is

    CXXStaticCastExpr 0x... 'struct S1' static_cast<struct S1> <ConstructorConversion>
    `-CXXConstructExpr 0x... 'struct S1' 'void (struct S1 &&) noexcept' elidable

    `-MaterializeTemporaryExpr 0x... 'struct S1' xvalue
      `-ImplicitCastExpr 0x... 'struct S1' <UserDefinedConversion>
        `-CXXMemberCallExpr 0x... 'struct S1'
          `-MemberExpr 0x... '<bound member function type>' .operator S1 0x...
            `-DeclRefExpr 0x... 'struct S2' lvalue ParmVar 0x... 's' 'struct S2'

getSubExprAsWritten used to return the MaterializeTemporaryExpr (of type S1)
under the CXXConstructExpr, instead of unwinding further to the DeclRefExpr (of
type S2) at the bottom.

Differential Revision: https://reviews.llvm.org/D22128

llvm-svn: 306377