Vulkan: Finish semantics for what creates spec-const-semantics.

Note: This required adding a new test mode to see the AST for vulkan tests.
This also required reworking some deeper parts of type creation, regarding
when storage qualification and constness is deduced bottom-up or dictated
top-down.

gtests/Spv.FromFile.cpp

@@ -43,6 +43,7 @@ namespace {
 
 using CompileToSpirvTest = GlslangTest<::testing::TestWithParam<std::string>>;
 using VulkanSemantics = GlslangTest<::testing::TestWithParam<std::string>>;
+using VulkanAstSemantics = GlslangTest<::testing::TestWithParam<std::string>>;
 
 // Compiling GLSL to SPIR-V under Vulkan semantics. Expected to successfully
 // generate SPIR-V.
@@ -62,6 +63,14 @@ TEST_P(VulkanSemantics, FromFile)
                             Target::Spv);
 }
 
+// GLSL-level Vulkan semantics test that need to see the AST for validation.
+TEST_P(VulkanAstSemantics, FromFile)
+{
+    loadFileCompileAndCheck(GLSLANG_TEST_DIRECTORY, GetParam(),
+                            Source::GLSL, Semantics::Vulkan,
+                            Target::AST);
+}
+
 // clang-format off
 INSTANTIATE_TEST_CASE_P(
     Glsl, CompileToSpirvTest,
@@ -187,6 +196,14 @@ INSTANTIATE_TEST_CASE_P(
     })),
     FileNameAsCustomTestSuffix
 );
+
+INSTANTIATE_TEST_CASE_P(
+    Glsl, VulkanAstSemantics,
+    ::testing::ValuesIn(std::vector<std::string>({
+        "vulkan.ast.vert",
+    })),
+    FileNameAsCustomTestSuffix
+);
 // clang-format on
 
 }  // anonymous namespace