glsl_shader_decompiler: Use std::string_view instead of std::string for AddLine()

This function doesn't need to take ownership of the string data being
given to it, considering all we do is append the characters to the
internal string instance.

Instead, use a string view to simply reference the string data without
any potential heap allocation.

Now anything that is a raw const char* won't need to be converted to a
std::string before appending.