Emit an extension warning when changing system header tokens

clang converts keywords to identifiers for compatibility with various system
headers such as GNU libc.

Implement a -Wkeyword-compat extension warning to diagnose those cases. The
warning is on by default but will generally be ignored in system headers. It
can however be enabled globally to aid standards conformance testing.

This also changes the __uptr keyword avoidance from r195710 to no longer
special-case system headers, bringing it in line with other similar workarounds
in clang.

Implementation returns bool for symmetry with token annotation functions.

Some examples:

warning: keyword '__is_pod' will be treated as an identifier for the remainder of the translation unit [-Wkeyword-compat]
struct __is_pod

warning: keyword '__uptr' will be treated as an identifier here [-Wkeyword-compat]
union w *__uptr;