[ubsan] Fix __ubsan_on_report interface definition

Speculative fix for the interface definition of __ubsan_on_report for
the Windows bots:

  http://lab.llvm.org:8011/builders/sanitizer-windows/builds/30528
  lib\ubsan\ubsan_interface.inc(55): error C2065: '__ubsan_on_report':
  undeclared identifier

INTERCEPT_SANITIZER_WEAK_FUNCTION was the wrong macro to use to begin
with because __ubsan_on_report isn't weak. Reading through that macro,
it's still not clear to me why there is an undefined reference, though,
because it appears to define a dummy __ubsan_on_report shim.

llvm-svn: 335383