[Request-evaluator] Add PrettyStackTrace support.

Print out the stack of requests being evaluated when we crash, so we can see
the dependency chain the triggered the crash.

Author: DougGregor

include/swift/AST/Evaluator.h

@@ -24,6 +24,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/Support/PrettyStackTrace.h"
 #include <string>
 #include <tuple>
 #include <type_traits>
@@ -40,6 +41,21 @@ using llvm::None;
 
 class DiagnosticEngine;
 
+/// Pretty stack trace handler for an arbitrary request.
+template<typename Request>
+class PrettyStackTraceRequest : public llvm::PrettyStackTraceEntry {
+  const Request &request;
+
+public:
+  PrettyStackTraceRequest(const Request &request) : request(request) { }
+
+  void print(llvm::raw_ostream &out) const {
+    out << "While evaluating request ";
+    simple_display(out, request);
+    out << "\n";
+  }
+};
+
 /// Evaluation engine that evaluates and caches "requests", checking for cyclic
 /// dependencies along the way.
 ///
@@ -210,6 +226,7 @@ class Evaluator {
     // them now anyway.
     dependencies[request].clear();
 
+    PrettyStackTraceRequest<Request> prettyStackTrace(request);
     return request(*this);
   }