Allocate MultipleDC objects using the allocator associated with

ASTContext instead of malloc.  Besides reducing malloc traffic, this
also removes a source of memory leaks when using a BumpPtrAllocator
for the allocator of ASTContext.  There are still leaks when using
MallocAllocator because Decl::Destroy() isn't fully finished.

Fixes: <rdar://problem/7431556>
llvm-svn: 90174