Don't emit duplicate decorations. (#3635)

It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.

Fixes #3627

gtests/Spv.FromFile.cpp

@@ -551,6 +551,7 @@ INSTANTIATE_TEST_SUITE_P(
         "spv.floatFetch.frag",
         "spv.atomicRvalue.error.vert",
         "spv.sampledImageBlock.frag",
+        "spv.multiple.var.same.const.frag",
     })),
     FileNameAsCustomTestSuffix
 );