Increase the macro id cache to look up several recent entries, not just the last...

Increase the macro id cache to look up several recent entries, not just the last one.
This is important in insane cases like the one dannyb sent me recently:

#define F0(a) void a(){}
#define F1(a) F0(a##0) F0(a##1) F0(a##2) F0(a##3) F0(a##4) F0(a##5) F0(a##6) F0(a##7)
#define F2(a) F1(a##0) F1(a##1) F1(a##2) F1(a##3) F1(a##4) F1(a##5) F1(a##6) F1(a##7)
#define F3(a) F2(a##0) F2(a##1) F2(a##2) F2(a##3) F2(a##4) F2(a##5) F2(a##6) F2(a##7)
#define F4(a) F3(a##0) F3(a##1) F3(a##2) F3(a##3) F3(a##4) F3(a##5) F3(a##6) F3(a##7)
#define F5(a) F4(a##0) F4(a##1) F4(a##2) F4(a##3) F4(a##4) F4(a##5) F4(a##6) F4(a##7)
#define F6(a) F5(a##0) F5(a##1) F5(a##2) F5(a##3) F5(a##4) F5(a##5) F5(a##6) F5(a##7)
F6(f)

cpp is great.  :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@40715 91177308-0d34-0410-b5e6-96231b3b80d8