i#2876 clang-format: check format in test suite

[derekbruening]

Adds checking of diff formatting to runsuite.cmake. If
clang-format-diff{,.py} is found, it is used to check the formatting, and
any format change is a fatal error. On Travis, not finding the program is
a fatal error.

Adds installation of clang-format-6.0 from apt.llvm.org on Travis.

Issue: #2876

[derekbruening]

I don't understand the AArch64 failure -- it must be getting the wrong diff, not just of this patch but the already-committed one?

CMake Error at /data/jenkins_root/workspace/DynamoRIO-AArch64-Precommit/suite/runsuite.cmake:235 (message):
  ERROR: diff contains tabs:

  -#define VC_EXTRALEAN		// Exclude rarely-used stuff from Windows headers

[derekbruening]

It worked. Appveyor failed due to nudge_test (#2978) and drmgr-test (#653). Travis failed due to the threadfilter test (#2941). We should add these to the ignore list (even better to fix of course...)

Now to test a deliberate format error. I tested this locally of course, but to ensure it works on Travis.