start gather stats on types processed. carbon.h currently yields:

*** AST Context Stats:
  30594 types total.
    19 builtin types
    3929 pointer types
    308 array types
    18883 function types with proto
    8 function types with no proto
    2988 typename (typedef) types
    4459 tagged types
      1476 struct types
      80 union types
      0 class types
      2903 enum types
  83298 slow type lookups

Next up, making type canonicalization not trivially silly.

llvm-svn: 39305