Finish making AST BumpPtrAllocation runtime configurable (based on -disable-free).

snaroff% time ../../Release-Asserts/bin/clang INPUTS/Cocoa_h.m
0.179u 0.051s 0:00.23 95.6%	0+0k 0+0io 0pf+0w
snaroff% time ../../Release-Asserts/bin/clang INPUTS/Cocoa_h.m -disable-free
0.169u 0.052s 0:00.22 95.4%	0+0k 0+0io 0pf+0w

llvm-svn: 63153