[CodeGen] Implement isTriviallyRecursive with StmtVisitor instead of RecursiveASTVisitor

This code doesn't need to traverse types, lambdas, template arguments,
etc to detect trivial recursion. We can do a basic statement traversal
instead. This reduces the time spent compiling CodeGenModule.cpp, the
object file size (mostly reduced debug info), and the final executable
size by a small amount. I measured the exe mostly to check how much of
the overhead is from debug info, object file section headers, etc, vs
actual code.

metric   | before | after | diff
time (s) | 47.4   | 38.5  | -8.9
obj (kb) | 12888  | 12012 | -876
exe (kb) | 86072  | 85996 | -76

llvm-svn: 352232