Relocate the external headers provided by ASan and the common sanitizer

Relocate the external headers provided by ASan and the common sanitizer
library.

These headers are intended to be available to user code when built with
AddressSanitizer (or one of the other sanitizer's in the future) to
interface with the runtime library. As such, they form stable external
C interfaces, and the headers shouldn't be located within the
implementation.

I've pulled them out into what seem like fairly obvious locations and
names, but I'm wide open to further bikeshedding of these names and
locations.

I've updated the code and the build system to cope with the new
locations, both CMake and Makefile. Please let me know if this breaks
anyone's build.

The eventual goal is to install these headers along side the Clang
builtin headers when we build the ASan runtime and install it. My
current thinking is to locate them at:

  <prefix>/lib/clang/X.Y/include/sanitizer/common_interface_defs.h
  <prefix>/lib/clang/X.Y/include/sanitizer/asan_interface.h
  <prefix>/lib/clang/X.Y/include/sanitizer/...

But maybe others have different suggestions?

Fixing the style of the #include between these headers at least unblocks
experimentation with installing them as they now should work when
installed in these locations.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@162822 91177308-0d34-0410-b5e6-96231b3b80d8