Made the ClangASTImporter into a shared pointer, eliminating a race condition.

It used to be a unique pointer, and there could be a case where ClangASTSource
held onto a copy of the pointer but Target::Destroy destroyed the unique pointer
in the mean time.

I also ensured that there is a validity check on the target (which confirms that
a ClangASTImporter can be generated) before the target's shared pointer is
copied into ClangASTSource.

This race condition caused a crash if Target::Destroy was called and then later
the target objecct was deleted.

llvm-svn: 252665