Flatten uniform arrays

This checkin adds a --flatten-uniform-arrays option which can break
uniform arrays of samplers, textures, or UBOs up into individual
scalars named (e.g) myarray[0], myarray[1], etc.  These appear as
individual linkage objects.

Code notes:

- shouldFlatten internally calls shouldFlattenIO, and shouldFlattenUniform,
  but is the only flattening query directly called.

- flattenVariable will handle structs or arrays (but not yet arrayed structs;
  this is tested an an error is generated).

- There's some error checking around unhandled situations.  E.g, flattening
  uniform arrays with initializer lists is not implemented.

- This piggybacks on as much of the existing mechanism for struct flattening
  as it can.  E.g, it uses the same flattenMap, and the same
  flattenAccess() method.

- handleAssign() has been generalized to cope with either structs or arrays.

- Extended test infrastructure to test flattening ability.

gtests/Hlsl.FromFile.cpp

@@ -58,6 +58,7 @@ std::string FileNameAsCustomTestSuffix(
 }
 
 using HlslCompileTest = GlslangTest<::testing::TestWithParam<FileNameEntryPointPair>>;
+using HlslCompileAndFlattenTest = GlslangTest<::testing::TestWithParam<FileNameEntryPointPair>>;
 
 // Compiling HLSL to SPIR-V under Vulkan semantics. Expected to successfully
 // generate both AST and SPIR-V.
@@ -68,6 +69,13 @@ TEST_P(HlslCompileTest, FromFile)
                             Target::BothASTAndSpv, GetParam().entryPoint);
 }
 
+TEST_P(HlslCompileAndFlattenTest, FromFile)
+{
+    loadFileCompileFlattenUniformsAndCheck(GLSLANG_TEST_DIRECTORY, GetParam().fileName,
+                                           Source::HLSL, Semantics::Vulkan,
+                                           Target::BothASTAndSpv, GetParam().entryPoint);
+}
+
 // clang-format off
 INSTANTIATE_TEST_CASE_P(
     ToSpirv, HlslCompileTest,
@@ -181,5 +189,15 @@ INSTANTIATE_TEST_CASE_P(
 );
 // clang-format on
 
+// clang-format off
+INSTANTIATE_TEST_CASE_P(
+    ToSpirv, HlslCompileAndFlattenTest,
+    ::testing::ValuesIn(std::vector<FileNameEntryPointPair>{
+        {"hlsl.array.flatten.frag", "main"},
+    }),
+    FileNameAsCustomTestSuffix
+);
+
+// clang-format on
 }  // anonymous namespace
 }  // namespace glslangtest