implement trivial scope caching. This reduces malloc traffic in the common

case, speeding up parsing of this contrived example:

#define A {{}}
#define B A A A A A A A A A A
#define C B B B B B B B B B B
#define D C C C C C C C C C C
#define E D D D D D D D D D D
#define F E E E E E E E E E E
#define G F F F F F F F F F F
#define H G G G G G G G G G G

void foo() {
  H
}

from 7.478s to 4.321s.  GCC requires 8.2s.

llvm-svn: 39138