Eliminate memory allocation from most invocations of

Eliminate memory allocation from most invocations of
ModuleManager::visit() by keeping a free list of the two data
structures used to store state (a preallocated stack and a visitation
number vector). Improves -fsyntax-only performance for my modules test
case by 2.8%. Modules has pulled ahead by almost 10% with the global
module index.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173692 91177308-0d34-0410-b5e6-96231b3b80d8