[ubsan] Use the correct tool name in diagnostics

When using ASan and UBSan together, the common sanitizer tool name is
set to "AddressSanitizer". That means that when a UBSan diagnostic is
printed out, it looks like this:

  SUMMARY: AddressSanitizer: ...

This can confuse users. Fix it so that we always use the correct tool
name when printing out UBSan diagnostics.

Differential Revision: https://reviews.llvm.org/D32066

llvm-svn: 300358