Make crash diagnostics on Windows the tiniest bit more useful

This escapes any backslashes in the executable path and fixes an issue
with a trailing quote when the main file name had to be quoted during
printing.

It's impossible to test this without putting backslashes or quotes into
the executable path, so I didn't add automated tests.

The crash diagnostics are still only useful if you're using bash on
Windows, though.  This should probably be writing a batch file instead.

llvm-svn: 214924