swift234

SILGen: Don't try to write back to rvalue decl or member refs.

Making DeclRefExprs and MemberRefExprs work as lvalues or rvalues simplifies the AST, but eliminates LoadExpr as a nice single focal point for severing writeback chains. We need to disable writeback scopes when codegenning rvalue DeclRefs and MemberRefs too. Fixes <rdar://problem/16530235>.

Swift SVN r15965