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.

llvm-svn: 91449