CMake: fix warnings on msvc builds

Remove /W3 from default flags, and disable a couple of warnings:

C4127: conditional expression is constant
C4458: declaration of 'identifier' hides class member

These match our warning settings of Clang/GCC more closely.
Also fix some valid warnings in some tests.

Change-Id: I46cb30b93ece74039db4aa0d6b52a675ee36859d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44960
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

CMakeLists.txt

@@ -76,6 +76,14 @@ message(STATUS "Tint build checking [chromium-style]: ${TINT_CHECK_CHROMIUM_STYL
 message(STATUS "Using python3")
 find_package(PythonInterp 3 REQUIRED)
 
+# CMake < 3.15 sets /W3 in CMAKE_CXX_FLAGS. Remove it if it's there.
+# See https://gitlab.kitware.com/cmake/cmake/-/issues/18317
+if (MSVC)
+    if (CMAKE_CXX_FLAGS MATCHES "/W3")
+        string(REPLACE "/W3" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+    endif()
+endif()
+
 if (${TINT_CHECK_CHROMIUM_STYLE})
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -add-plugin -Xclang find-bad-constructs")
 endif()
@@ -186,8 +194,10 @@ function(tint_default_compile_options TARGET)
       /W4
       /WX
       /wd4068
+      /wd4127
       /wd4244
       /wd4267
+      /wd4458
       /wd4514
       /wd4571
       /wd4625

src/writer/spirv/builder_discard_test.cc

@@ -28,7 +28,7 @@ TEST_F(BuilderTest, Discard) {
   spirv::Builder& b = Build();
 
   b.push_function(Function{});
-  EXPECT_EQ(b.GenerateStatement(expr), 1u) << b.error();
+  EXPECT_TRUE(b.GenerateStatement(expr)) << b.error();
   EXPECT_EQ(DumpInstructions(b.functions()[0].instructions()), R"(OpKill
 )");
 }

src/writer/spirv/builder_function_decoration_test.cc

@@ -65,7 +65,8 @@ TEST_P(Decoration_StageTest, Emit) {
   EXPECT_EQ(preamble[0].opcode(), spv::Op::OpEntryPoint);
 
   ASSERT_GE(preamble[0].operands().size(), 3u);
-  EXPECT_EQ(preamble[0].operands()[0].to_i(), params.model);
+  EXPECT_EQ(preamble[0].operands()[0].to_i(),
+            static_cast<uint32_t>(params.model));
 }
 INSTANTIATE_TEST_SUITE_P(
     BuilderTest,