Work around a valgrind oddity where it doesn't pass the full path of a

Work around a valgrind oddity where it doesn't pass the full path of a
script to the #! command by using bash instead of /bin/sh.  Bash
searches $PATH for its script argument, but dash, which /bin/sh
resolves to on some systems, does not.
https://bugs.kde.org/show_bug.cgi?id=231257 tracks the valgrind
problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98892 91177308-0d34-0410-b5e6-96231b3b80d8