-Wmicrosoft: Don't warn on non-inline pure virtual method definitions

MSVC and clang with -fms-extensions allow pure virtual methods to be
defined inline after the "= 0" tokens.  Clang warns on these because it
is not standard, but incorrectly warns on out-of-line definitions, which
are standard.

With this change, clang will only warn on inline definitions of pure
virtual methods.

Fixes some self-host warnings on out-of-line definitions of pure virtual
destructors.