Merge used flags so that we don't have to iterate on isUsed. With this change

Merge used flags so that we don't have to iterate on isUsed. With this change
"clang -cc1 -fsyntax-only" on the preprocessed output of

#define M extern int a;
#define M2 M M
#define M4 M2 M2
#define M8 M4 M4
#define M16 M8 M8
#define M32 M16 M16
#define M64 M32 M32
#define M128 M64 M64
#define M256 M128 M128
#define M512 M256 M256
#define M1024 M512 M512
#define M2048 M1024 M1024
#define M4096 M2048 M2048
#define M8192 M4096 M4096
#define M16384 M8192 M8192
M16384

goes from 2.994s to 1.416s. GCC is at 0.022s, so we still have a long way to go.

llvm-svn: 168519