Vulkan: Fix qualifiers for varyings

`out` variables in the vertex shader and `in` variables in the fragment
shader were not decorated with `layout(location=?)` as they should
according to the Vulkan GLSL spec.  This change makes sure these
decorations are present regardless of whether the compiler sees these
variables as EvqVaryingIn/Out or EvqVertexOut/FragmentIn.

Bug: angleproject:3412
Change-Id: I66473e876cb989a60b0c6d6a5850a8b2c763d8e2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1590694
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

src/compiler/translator/OutputVulkanGLSL.cpp

@@ -84,8 +84,7 @@ void TOutputVulkanGLSL::writeLayoutQualifier(TIntermTyped *variable)
 
 void TOutputVulkanGLSL::writeQualifier(TQualifier qualifier, const TSymbol *symbol)
 {
-    if (qualifier != EvqUniform && qualifier != EvqVaryingIn && qualifier != EvqVaryingOut &&
-        qualifier != EvqAttribute)
+    if (qualifier != EvqUniform && qualifier != EvqAttribute && !sh::IsVarying(qualifier))
     {
         TOutputGLSLBase::writeQualifier(qualifier, symbol);
         return;

src/tests/gl_tests/GLSLTest.cpp

@@ -3522,9 +3522,6 @@ TEST_P(GLSLTest_ES3, ComplexVaryingStructsUsedInFragmentShader)
 // Test that an inactive varying struct that doesn't get used in the fragment shader works.
 TEST_P(GLSLTest_ES3, InactiveVaryingStructUnusedInFragmentShader)
 {
-    // TODO(syoussefi): http://anglebug.com/3412
-    ANGLE_SKIP_TEST_IF(IsVulkan());
-
     constexpr char kVS[] =
         "#version 300 es\n"
         "in vec4 inputAttribute;\n"