Make 'LangOptions' in CompilerInvocation a heap-allocated, reference counted object...

Make 'LangOptions' in CompilerInvocation a heap-allocated, reference counted object.  I discovered that llvm::RefCountedBase<T> has
a bug where the reference count is copied in the copy constructor, which means that there were cases when the CompilerInvocation
objects created by ASTUnit were actually leaked.  When I fixed that bug locally, it showed that a whole bunch of code assumed
that the LangOptions object that was part of CompilerInvocation was still alive.  By making it heap-allocated and reference counted,
we can keep it around after the CompilerInvocation object goes away.

As part of this change, change CompilerInvocation:getLangOptions() to return a pointer, acting as another clue that this
object may outlive the CompilerInvocation object.

This commit doesn't fix the CompilerInvocation leak itself.  That will come when I commit the fix to llvm::RefCountedBase<T> to
mainline LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144930 91177308-0d34-0410-b5e6-96231b3b80d8