Introduce new AST statistics that keep track of the number of isa (or

dyn_cast) invocations for C++ and Objective-C types, declarations,
expressions, and statements. The statistics will be printed when
-print-stats is provided to Clang -cc1, with results such as:

277073 clang - Number of checks for C++ declaration nodes
 13311 clang - Number of checks for C++ expression nodes
    18 clang - Number of checks for C++ statement nodes
174182 clang - Number of checks for C++ type nodes
 92300 clang - Number of checks for Objective-C declaration nodes
  9800 clang - Number of checks for Objective-C expression nodes
     7 clang - Number of checks for Objective-C statement nodes
 65733 clang - Number of checks for Objective-C type nodes

The statistics are only gathered when NDEBUG is not defined, since
they introduce potentially-expensive operations into very low-level
routines (isa).

llvm-svn: 99912