[Diagnostics] Add a tailored diagnostic for Swift -> C pointer conversion

Diagnose situations where Swift -> C pointer implicit conversion
is attempted on a Swift function instead of one imported from C header.

```swift
func test(_: UnsafePointer<UInt8>) {}

func pass_ptr(ptr: UnsafeRawPointer) {
  test(ptr) // Only okay if `test` was an imported C function.
}
```

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -362,6 +362,11 @@ ERROR(cannot_convert_argument_value,none,
       "cannot convert value of type %0 to expected argument type %1",
       (Type,Type))
 
+ERROR(cannot_convert_argument_value_for_swift_func,none,
+      "cannot convert value of type %0 to expected argument type %1 "
+      "because %2 %3 was not imported from C header",
+      (Type,Type, DescriptiveDeclKind, DeclName))
+
 NOTE(candidate_has_invalid_argument_at_position,none,
      "candidate expects %select{|in-out }2value of type %0 for parameter #%1 (got %3)",
      (Type, unsigned, bool, Type))

lib/Sema/CSDiagnostics.cpp

@@ -7825,3 +7825,17 @@ bool InvalidWeakAttributeUse::diagnoseAsError() {
 
   return true;
 }
+
+bool SwiftToCPointerConversionInInvalidContext::diagnoseAsError() {
+  auto argInfo = getFunctionArgApplyInfo(getLocator());
+  if (!argInfo)
+    return false;
+
+  auto *callee = argInfo->getCallee();
+  auto argType = resolveType(argInfo->getArgType());
+  auto paramType = resolveType(argInfo->getParamType());
+
+  emitDiagnostic(diag::cannot_convert_argument_value_for_swift_func, argType,
+                 paramType, callee->getDescriptiveKind(), callee->getName());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -2607,6 +2607,26 @@ class InvalidWeakAttributeUse final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose situations where Swift -> C pointer implicit conversion
+/// is attempted on a Swift function instead of one imported from C header.
+///
+/// \code
+/// func test(_: UnsafePointer<UInt8>) {}
+///
+/// func pass_ptr(ptr: UnsafeRawPointer) {
+///   test(ptr) // Only okay if `test` was an imported C function.
+/// }
+/// \endcode
+class SwiftToCPointerConversionInInvalidContext final
+    : public FailureDiagnostic {
+public:
+  SwiftToCPointerConversionInInvalidContext(const Solution &solution,
+                                            ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2003,7 +2003,8 @@ AllowNonOptionalWeak *AllowNonOptionalWeak::create(ConstraintSystem &cs,
 
 bool AllowSwiftToCPointerConversion::diagnose(const Solution &solution,
                                               bool asNote) const {
-  return false;
+  SwiftToCPointerConversionInInvalidContext failure(solution, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AllowSwiftToCPointerConversion *