[CSDiagnostics] Add a diagnostic for missing each and provide a fix-it

Diagnose situations where value pack is referenced without an explicit 'each':

```
func compute<each T>(_: repeat each T) {}

func test<each T>(v: repeat each T) {
  repeat compute(v) // should be `repeat compute(each v)`
}
```

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5419,6 +5419,9 @@ ERROR(pack_reference_must_be_in_expansion,none,
 ERROR(pack_type_requires_keyword_each,none,
       "pack type %0 must be referenced with 'each'",
       (TypeRepr*))
+ERROR(value_pack_requires_keyword_each,none,
+      "value pack %0 must be referenced with 'each'",
+      (Type))
 ERROR(tuple_duplicate_label,none,
       "cannot create a tuple with a duplicate element label", ())
 ERROR(multiple_ellipsis_in_tuple,none,

lib/Sema/CSDiagnostics.cpp

@@ -8906,3 +8906,25 @@ bool ValuePackExpansionWithoutPackReferences::diagnoseAsError() {
   emitDiagnostic(diag::value_expansion_not_variadic);
   return true;
 }
+
+bool MissingEachForValuePackReference::diagnoseAsError() {
+  bool fixItNeedsParens = false;
+  // If 'each' is missing form a base of a member reference
+  // it has to be wrapped in parens.
+  if (auto anchor = getAsExpr(getAnchor())) {
+    fixItNeedsParens = isExpr<UnresolvedDotExpr>(findParentExpr(anchor));
+  }
+
+  {
+    auto diagnostic = emitDiagnostic(diag::value_pack_requires_keyword_each, ValuePackType);
+    if (fixItNeedsParens) {
+      auto range = getSourceRange();
+      diagnostic.fixItInsert(range.Start, "(each ")
+          .fixItInsertAfter(range.End, ")");
+    } else {
+      diagnostic.fixItInsert(getLoc(), "each ");
+    }
+  }
+
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -3022,6 +3022,27 @@ class ValuePackExpansionWithoutPackReferences final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose situations where value pack is referenced without explicit 'each':
+///
+/// \code
+/// func compute<each T>(_: repeat each T) {}
+///
+/// func test<each T>(v: repeat each T) {
+///   repeat compute(v) // should be `repeat compute(each v)`
+/// }
+/// \endcode
+class MissingEachForValuePackReference final : public FailureDiagnostic {
+  Type ValuePackType;
+
+public:
+  MissingEachForValuePackReference(const Solution &solution, Type valuePackTy,
+                                   ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator),
+        ValuePackType(resolveType(valuePackTy)) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift