P0188R1: add support for standard [[fallthrough]] attribute. This is almost

exactly the same as clang's existing [[clang::fallthrough]] attribute, which
has been updated to have the same semantics. The one significant difference
is that [[fallthrough]] is ill-formed if it's not used immediately before a
switch label (even when -Wimplicit-fallthrough is disabled). To support that,
we now build a CFG of any function that uses a '[[fallthrough]];' statement
to check.

In passing, fix some bugs with our support for statement attributes -- in
particular, diagnose their use on declarations, rather than asserting.

llvm-svn: 262881