transform/EmitVertexPointSize: Handle entry point parameters

Generate a new struct that contains members of the original return
type with the point size appended to it, and replace return statements
as necessary.

Fixed: tint:732
Change-Id: I2b5816144d5e95c65baca95dc0c50b4dfdd25ed3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48980
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/program_builder.h

@@ -700,6 +700,9 @@ class ProgramBuilder {
   // AST helper methods
   //////////////////////////////////////////////////////////////////////////////
 
+  /// @return a new unnamed symbol
+  Symbol Sym() { return Symbols().New(); }
+
   /// @param name the symbol string
   /// @return a Symbol with the given name
   Symbol Sym(const std::string& name) { return Symbols().Register(name); }