[AST] Disable comment printing while emitting diagnostics

There is a problem in PCH builds where source manager might
end up having unorderable source locations for comments, to work
around this (which is going to be fixed separately) let's
disable comment printing while emitting diagnostics since
such comments are not required.

Resolves: rdar://problem/38203776

Author: xedin

include/swift/AST/PrintOptions.h

@@ -402,6 +402,7 @@ struct PrintOptions {
     result.PrintIfConfig = false;
     result.ShouldQualifyNestedDeclarations =
         QualifyNestedDeclarations::TypesOnly;
+    result.PrintDocumentationComments = false;
     return result;
   }
 
@@ -418,6 +419,7 @@ struct PrintOptions {
     result.ElevateDocCommentFromConformance = true;
     result.ShouldQualifyNestedDeclarations =
         QualifyNestedDeclarations::Always;
+    result.PrintDocumentationComments = true;
     return result;
   }