Only analyse typeless inner expressions.

Cython uses many static inline utility functions.  Most compilers
don't emit warnings for such functions, but Clang does if they're
not defined in header files.  As such functions are often defined
as a group (e.g. utility code for a specific type) it would be
cumbersome to thin them out to those that happen to actually get
used.  Instead, simply suppress this warning for this compiler
for all inline functions.

This fixes #1523.

This closes #1522.

Move CYTHON_UNUSED macro up such that included files can use it

Closes #1506

Closes #1506

Fixes #1499

This gives projects more time to move over, and reduces the number
of changes in the 0.25 series.

Fixes #1502

Note about directive defaults.

Closes #1496

Closes #1493

This partially reverts 65d3b977,
but is more correct.

Closes #1492