[CrashReproducer][Darwin] Suggest attaching .crash diagnostic file

In addition to the preprocessed sources file and reproducer script, also
point to the .crash diagnostic files on Darwin. Example:

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-4.0: note: diagnostic msg: /var/folders/bk/1hj20g8j4xvdj5gd25ywhd3m0000gq/T/RegAllocGreedy-238f28.cpp
clang-4.0: note: diagnostic msg: /var/folders/bk/1hj20g8j4xvdj5gd25ywhd3m0000gq/T/RegAllocGreedy-238f28.cache
clang-4.0: note: diagnostic msg: /var/folders/bk/1hj20g8j4xvdj5gd25ywhd3m0000gq/T/RegAllocGreedy-238f28.sh
clang-4.0: note: diagnostic msg: /var/folders/bk/1hj20g8j4xvdj5gd25ywhd3m0000gq/T/RegAllocGreedy-238f28.crash

When no match is found for the .crash, point the user to a directory
where those can be found. Example:

clang-4.0: note: diagnostic msg: Crash backtrace is located in
clang-4.0: note: diagnostic msg: /Users/bruno/Library/Logs/DiagnosticReports/clang-4.0_<YYYY-MM-DD-HHMMSS>_<hostname>.crash
clang-4.0: note: diagnostic msg: (choose the .crash file that corresponds to your crash)

rdar://problem/27286266

llvm-svn: 287262