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/Software/SWRenderer.cpp

@@ -68,7 +68,7 @@ public:
 };
 
 std::unique_ptr<AbstractShader>
-SWRenderer::CreateShaderFromSource(ShaderStage stage, const char* source, size_t length)
+SWRenderer::CreateShaderFromSource(ShaderStage stage, [[maybe_unused]] std::string_view source)
 {
   return std::make_unique<SWShader>(stage);
 }