ASTContext: Don't use nothrow specifications on the new operators for allocating

from the ASTContext.
 - Doing so requires the compiler to generate null checks against the returned
   result, but the BumpPtrAllocator never returns null pointers.
 - The optimizer can usually eliminate such checks, but not always, so this
   gives us tighter codegen in some places.
 - It would be really nice if we could just use __builtin_unreachable or
   something to tell the optimizer that the allocator never returns null, but
   LLVM isn't currently that smart.

llvm-svn: 152060