Add -Winfinite-recursion to Clang

This new warning detects when a function will recursively call itself on every
code path though that function.  This catches simple recursive cases such as:

void foo() {
  foo();
}

As well as more complex functions like:

void bar() {
  if (test()) {
    bar();
    return;
  } else {
    bar();
  }
  return;
}

This warning uses the CFG.  As with other CFG-based warnings, this is off
by default.  Due to false positives, this warning is also disabled for
templated functions.

llvm-svn: 197853