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