Always check result of Thread::ModifySuspendCount

Ensures that we never ignore the result of ModifySuspendCount so that
we can react if the suspend count is not updated as expected.

This CL does the following:
* Adds __attribute__((warn_unused_result)) on the method to raise an
  error at compilation time if the result is ignored.
* Wraps calls with DCHECK where the result used to be ignored.

Bug: 27385848
Test: make -j test-art-host
Test: art/tools/run-jdwp-tests.sh --mode=host --variant=X64
Change-Id: I2d0e1ab7158c70ec8076c8bae6e4b814aee75af6