Make crash diagnostics on Windows the tiniest bit more useful

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214924 91177308-0d34-0410-b5e6-96231b3b80d8