Cleanup warnings of -Wmissing-declarations

[howard0su]

Add static to the functions which is not intentionally
export to big scope.

[stenzek]

LGTM once the lint issues are fixed. We could probably move the surrounding classes into an anonymous namespace too since they're not used outside, but happy for that to be a separate change.

Format fix:

!!! Source/Core/VideoBackends/Vulkan/ShaderCompiler.cpp not compliant to coding style, here is the fix:
--- Source/Core/VideoBackends/Vulkan/ShaderCompiler.cpp	2020-03-23 18:18:38.764021444 +0000
+++ -	2020-03-23 18:18:41.440714291 +0000
@@ -107,8 +107,10 @@
   #define SUBGROUP_MAX(value) value = subgroupMax(value)
 )";
 
-static std::optional<SPIRVCodeVector> CompileShaderToSPV(EShLanguage stage, const char* stage_filename,
-                                                  std::string_view source, std::string_view header)
+static std::optional<SPIRVCodeVector> CompileShaderToSPV(EShLanguage stage,
+                                                         const char* stage_filename,
+                                                         std::string_view source,
+                                                         std::string_view header)
 {
   if (!InitializeGlslang())
     return std::nullopt;