Remove some uses of obsolete 'register' storage specifier

Remove the keyword from all Source/* files outside of KWSys.
Document removal of 'register' from flex/bison output

In each .l and .y input file that has instructions for manual
transformation of the output to include a step to remove the 'register'
storage specifier.
Merge topic 'clang-register-warnings' into next

e1381b1 Document removal of 'register' from flex/bison output
1399825 Remove some uses of obsolete 'register' storage specifier