[sanitizer] Further split private and public sanitizer headers.

And make msan_interface.h C-compatible.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173928 91177308-0d34-0410-b5e6-96231b3b80d8
[Sanitizer] Add change missed in r173926

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173929 91177308-0d34-0410-b5e6-96231b3b80d8