Expressions can run without a process.

Code was added in ClangExpressionParser::ClangExpressionParser that was calling through
the process w/o checking that it was good.  Also, we were pretending that we could do something
reasonable if we had no target, but that's actually not true, so I check for a target at the
beginning of the constructor and don't make a compiler in that case.

<rdar://problem/25841198>

llvm-svn: 266944