VideoCommon/RenderBase: Use a std::string_view with CreateShaderFromSource()

Greatly simplifies the overall interface when it comes to compiling
shaders. Also allows getting rid of a std::string overload of the same
name. Now std::string and const char* both go through the same function.

Source/Core/VideoBackends/Null/Render.cpp

@@ -48,8 +48,8 @@ public:
   ~NullShader() = default;
 };
 
-std::unique_ptr<AbstractShader> Renderer::CreateShaderFromSource(ShaderStage stage,
-                                                                 const char* source, size_t length)
+std::unique_ptr<AbstractShader>
+Renderer::CreateShaderFromSource(ShaderStage stage, [[maybe_unused]] std::string_view source)
 {
   return std::make_unique<NullShader>(stage);
 }