[NFC] Removed Explosion::peek.

The convenience was added to aid with the async calling convention but
it has proven to be unnecessary.  Here, it is removed.

Author: nate-chandler

lib/IRGen/Explosion.h

@@ -152,11 +152,6 @@ class Explosion {
     return Values[NextValue-1];
   }
 
-  llvm::Value *peek(unsigned n) {
-    assert(n < Values.size());
-    return Values[n];
-  }
-
   /// Claim and remove the last item in the array.
   /// Unlike the normal 'claim' methods, the item is gone forever.
   llvm::Value *takeLast() {