[AST] Memoize ASTContext::getTypeInfo().

 - On -emit-llvm-only of 403.gcc/combine.c, for example, we make 160k calls to
   getTypeInfo but only ever deal with 680 some distinct types.

I saw these speedups (user time):
  403.gcc/combine.c -- 3.1%
  OmniGroupFrameworks/NSBezierPath-OAExtensions.m -- 3.6%
  JavaScriptCore/Interpreter.cpp -- 1.4%
which seems pretty sweet.

I ran some histograms on those compiles and we end up doing a ton of
getTypeInfo() on 'char' and 'int'. I tried splitting out a fast path for builtin
types, but this wasn't a win. Still kinda seems like we could be doing better
here.

llvm-svn: 152377