Add option to validate after each pass (#2462)

* New command-line option to opt: --validate-after-all
 * Pass manager will validate after each pass it runs
diff --git a/include/spirv-tools/optimizer.hpp b/include/spirv-tools/optimizer.hpp
index 08ef5e6..adfd492 100644
--- a/include/spirv-tools/optimizer.hpp
+++ b/include/spirv-tools/optimizer.hpp
@@ -199,6 +199,9 @@
   // |out| output stream.
   Optimizer& SetTimeReport(std::ostream* out);
 
+  // Sets the option to validate the module after each pass.
+  Optimizer& SetValidateAfterAll(bool validate);
+
  private:
   struct Impl;                  // Opaque struct for holding internal data.
   std::unique_ptr<Impl> impl_;  // Unique pointer to internal data.