DiagnoseInfiniteRecursion: handle invariant conditions and improve the warning message

The main change is to detect infinite recursive calls under invariant conditions. For example:

  func f() {
    if #available(macOS 10.4.4, *) {
      f()
    }
  }

or invariant conditions due to forwarded arguments:

  func f(_ x: Int) {
    if x > 0 {
      f(x)
    }
  }

Also, improve the warning message. Instead of giving a warning at the function location

  warning: all paths through this function will call itself

give a warning at the call location:

  warning: function call causes an infinite recursion

Especially in case of multiple recursive calls, it makes it easier to locate the problem.

https://bugs.swift.org/browse/SR-11842
rdar://57460599

Author: eeckstein

include/swift/AST/DiagnosticsSIL.def

@@ -277,8 +277,8 @@ WARNING(unreachable_case,none,
 WARNING(switch_on_a_constant,none,
         "switch condition evaluates to a constant", ())
 NOTE(unreachable_code_note,none, "will never be executed", ())
-WARNING(warn_infinite_recursive_function,none,
-        "all paths through this function will call itself", ())
+WARNING(warn_infinite_recursive_call,none,
+        "function call causes an infinite recursion", ())
 
 // 'transparent' diagnostics
 ERROR(circular_transparent,none,

include/swift/SIL/SILBasicBlock.h

@@ -314,6 +314,10 @@ public llvm::ilist_node<SILBasicBlock>, public SILAllocated<SILBasicBlock> {
   const_succblock_iterator succblock_end() const {
     return getTerminator()->succblock_end();
   }
+  
+  unsigned getNumSuccessors() const {
+    return getTerminator()->getNumSuccessors();
+  }
 
   SILBasicBlock *getSingleSuccessorBlock() {
     return getTerminator()->getSingleSuccessorBlock();