Add cmake option to disable minidumps, default it to off

Since crash dumping landed in r268519, May 2016, I have not once seen
anyone use an uploaded minidump to debug a compiler crash. Therefore,
I'm turning this off by default. The dumps clutter up user and buildbot
temp directories. Each file is only about 56KB, but it adds up.

In the context of clang, the extra line about the minidump confuses
users, when what we really want from them is the pre-processed source
code.

llvm-svn: 340185