Prevent using built-ins for func names in shaders

[Chaosus]

Using built-in functions for function names are incorrect - the code won't allow calling such function (and it won't in GLSL):

but currently it will not emit error if user not calling such function:

after this fix it will:

Note: other use cases for built-in names such as constants, variables and function arguments are still be valid and untouched.

[akien-mga]

Thanks!