Cherry-pick the enforce_tcb attribute.

[haoNoQ]

Introduce a function attribute 'enforce_tcb' that prevents the function
from calling other functions without the same attribute. This allows
isolating code that's considered to be somehow privileged so that it could not
use its privileges to exhibit arbitrary behavior.

Introduce an on-by-default warning '-Wtcb-enforcement' that warns
about violations of the above rule.

Introduce a function attribute 'enforce_tcb_leaf' that suppresses
the new warning within the function it is attached to. Such leaf functions
may implement common functionality between the trusted and the untrusted code
but they require extra careful audit with respect to their capabilities.

Fixes after a revert in 419ef38:
Fix a test.
Add workaround for GCC bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67274).
Attribute the patch appropriately!

Differential Revision: https://reviews.llvm.org/D91898

(cherry picked from commit 35c9baa)

[haoNoQ]

@swift-ci test

[SavchenkoValeriy]

Uh-oh, I kinda missed the resolution of this change getting reverted, can you please point it out?

[haoNoQ]

This is already the fixed patch that was re-committed after the revert. I fixed the test and worked around the gcc bug on one of the buildbots. I didn't cherry-pick the reverted patch (c163aae) or the revert commit (419ef38).

[haoNoQ]

The fix for the gcc error is here. I had to say this->Diag instead of just Diag because gcc before 7.1 is buggy.

[SavchenkoValeriy]

LGTM!
(Let's still wait for the Mac bot)