Rewrite removeVarsAndFuncs.

The current version has these bugs:
- Adding a semicolon after a function results in the removal of structures
  following the function.
- Function like macros get removed on accident rather than on purpose.
- It removes extern "C" { completely, which might not be a bug, but doesn't
  seem right.

I couldn't easily fix any of these problems because the code depends heavily
on the header being correct.

New unit tests added for the function to cover all of these cases.

A follow-on CL will include the updated headers.

Bug: 112290385

Test: Passes all new unit tests.
Test: When run on the current kernel headers, the generated headers are
Test: nearly the same, missing data is being added.
Change-Id: Ib22a5f2e78873544e8a9d54e385af1156b2a72bb