Clang plugin: added pure_tessera_op and globals for byref types

[jessicacotturone21]

Now Clang can parse something like this:
__attribute__((noinline, pure_tessera_op("eigen.inv(x:byref,y)")))
and will create global variables for each argument marked byref (in this case, x) so that we have access to their types later when loading in their values