build: raise warning level to 2 for MSVC + WX (KhronosGroup#6290)

SPIRV-Tools consumers like DXC build with MSVC using /W2. 
Adding this flag here to the CI can catch those warnings earlier. 
Also adding /WX to fail on warnings so those become visible on the CI.

This PR also disables the warning [4251](https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4251?view=msvc-170) which warns about
`std::unique_ptr` being in a class marked as dllexport.

Author: Keenuts

CMakeLists.txt

@@ -112,7 +112,7 @@ if(${COMPILER_IS_LIKE_GNU})
     set(SPIRV_WARNINGS ${SPIRV_WARNINGS} -Werror)
   endif()
 elseif(MSVC)
-  set(SPIRV_WARNINGS -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_WARNINGS /wd4800 /wd4819)
+  set(SPIRV_WARNINGS -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_WARNINGS /wd4800 /wd4819 /wd4251 /W2 /WX)
 
   if(${SPIRV_WERROR})
     set(SPIRV_WARNINGS ${SPIRV_WARNINGS} /WX)

test/opt/fold_test.cpp

@@ -1444,7 +1444,7 @@ ::testing::Values(
           "%2 = OpSNegate %v2uint %v2uint_0x3f800000_0xbf800000\n" +
           "OpReturn\n" +
           "OpFunctionEnd",
-      2, {static_cast<uint32_t>(-0x3f800000), static_cast<uint32_t>(-0xbf800000)}),
+      2, {static_cast<uint32_t>(-0x3f800000), static_cast<uint32_t>(-0xbf800000ll)}),
     // Test case 6: fold vector components of uint (including integer overflow)
     InstructionFoldingCase<std::vector<uint32_t>>(
       Header() + "%main = OpFunction %void None %void_func\n" +