[DiagnosticVerifier] Do not capture 'fatal error encountered' errors (#84640)

When the diagnostic verifier encounters a fatal error, it reports:

    <unknown>:0: error: fatal error encountered while in -verify mode

If we capture this, and fail to match this against an expected-error,
the diagnostic verifier complains:

    <unknown>:0: error: unexpected error produced: fatal error encountered while in -verify mode
    <unknown>:0: error: diagnostic produced elsewhere: fatal error encountered while in -verify mode

The current workaround is to use -verify-ignore-unknown, but that in
turn may mask actual errors coming from unknown source locations.
Ignoring these errors about errors removes the need for that workaround.

Author: j-hui

lib/Frontend/DiagnosticVerifier.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/Frontend/DiagnosticVerifier.h"
+#include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/Basic/Assertions.h"
 #include "swift/Basic/ColorUtils.h"
 #include "swift/Basic/SourceManager.h"
@@ -1454,6 +1455,10 @@ processExpansions(SourceManager &SM, llvm::DenseMap<SourceLoc, unsigned> &Expans
 /// file.
 void DiagnosticVerifier::handleDiagnostic(SourceManager &SM,
                                           const DiagnosticInfo &Info) {
+  // Ignore "fatal error encountered while in -verify mode" errors,
+  // because there's no reason to verify them.
+  if (Info.ID == diag::verify_encountered_fatal.ID)
+    return;
   SmallVector<CapturedFixItInfo, 2> fixIts;
   for (const auto &fixIt : Info.FixIts) {
     fixIts.emplace_back(SM, fixIt);

test/Frontend/DiagnosticVerifier/expect-the-expected.swift

@@ -0,0 +1,3 @@
+// RUN: %target-typecheck-verify-swift
+
+import Expected // expected-error {{no such module 'Expected'}}