Fix two classes of bugs. First:

validate an invariant so that the asmparser rejects a bad construct
instead of the verifier.  Before:

llvm-as: assembly parsed, but does not verify as correct!
Invalid struct return type!
i64 (%struct.Type*, %struct.Type*)* @foo

after:

llvm-as: t.ll:5:8: functions with 'sret' argument must return void
define i64 @foo(%struct.Type* noalias nocapture sret %agg.result, %struct.Type* nocapture byval %t) nounwind {
       ^


Second, check that void is only used where allowed (in function return types) not in
arbitrary places, fixing PR3747 - Crash in llvm-as with void field in struct.  We
now reject that example with:

$ llvm-as t.ll
llvm-as: t.ll:1:12: struct element can not have void type
%x = type {void}
           ^

llvm-svn: 66394