enable reuse of MacroArgs objects. This is a small (2.5%) win

enable reuse of MacroArgs objects.  This is a small (2.5%) win
on PR5610 (2.185 -> 2.130s).  The big issue is that this is making
insanely huge macro argument lists with over a million tokens in it.
The reason that mallco and free are so expensive is that we are
actually going to the kernel to get it, and switching to a bump
pointer allocator won't change this in an interesting way.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91449 91177308-0d34-0410-b5e6-96231b3b80d8