[clang-tidy] Fix broken HeaderFilterRegex when read from config file

PR llvm#91400 broke the usage
of HeaderFilterRegex via config file, because it is now created at a
different point in the execution and leads to a different value.

The result of that is that using HeaderFilterRegex only in the config
file does NOT work, in other words clang-tidy stops triggering warnings
on header files, thereby losing a lot of coverage.

This patch reverts the logic so that the header filter is created
upon calling the getHeaderFilter() function.

Additionally, this patch adds 2 unit tests to prevent regressions in
the future:

- One of them, "simple", tests the most basic use case with a single
  top-level .clang-tidy file.

- The second one, "inheritance", demonstrates that the subfolder only
  gets warnings from headers within it, and not from parent headers.

Fixes llvm#118009, llvm#121969, llvm#133453

Author: Carlos Gálvez

clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp

@@ -571,17 +571,30 @@ void ClangTidyDiagnosticConsumer::checkFilters(SourceLocation Location,
   }
 
   StringRef FileName(File->getName());
-  LastErrorRelatesToUserCode =
-      LastErrorRelatesToUserCode || Sources.isInMainFile(Location) ||
-      (HeaderFilter &&
-       (HeaderFilter->match(FileName) &&
-        !(ExcludeHeaderFilter && ExcludeHeaderFilter->match(FileName))));
+  LastErrorRelatesToUserCode = LastErrorRelatesToUserCode ||
+                               Sources.isInMainFile(Location) ||
+                               (getHeaderFilter()->match(FileName) &&
+                                !getExcludeHeaderFilter()->match(FileName));
 
   unsigned LineNumber = Sources.getExpansionLineNumber(Location);
   LastErrorPassesLineFilter =
       LastErrorPassesLineFilter || passesLineFilter(FileName, LineNumber);
 }
 
+llvm::Regex *ClangTidyDiagnosticConsumer::getHeaderFilter() {
+  if (!HeaderFilter)
+    HeaderFilter =
+        std::make_unique<llvm::Regex>(*Context.getOptions().HeaderFilterRegex);
+  return HeaderFilter.get();
+}
+
+llvm::Regex *ClangTidyDiagnosticConsumer::getExcludeHeaderFilter() {
+  if (!ExcludeHeaderFilter)
+    ExcludeHeaderFilter = std::make_unique<llvm::Regex>(
+        *Context.getOptions().ExcludeHeaderFilterRegex);
+  return ExcludeHeaderFilter.get();
+}
+
 void ClangTidyDiagnosticConsumer::removeIncompatibleErrors() {
   // Each error is modelled as the set of intervals in which it applies
   // replacements. To detect overlapping replacements, we use a sweep line

clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.h

@@ -302,6 +302,10 @@ class ClangTidyDiagnosticConsumer : public DiagnosticConsumer {
   /// context.
   llvm::Regex *getHeaderFilter();
 
+  /// Returns the \c ExcludeHeaderFilter constructed for the options set in the
+  /// context.
+  llvm::Regex *getExcludeHeaderFilter();
+
   /// Updates \c LastErrorRelatesToUserCode and LastErrorPassesLineFilter
   /// according to the diagnostic \p Location.
   void checkFilters(SourceLocation Location, const SourceManager &Sources);

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/.clang-tidy

@@ -0,0 +1 @@
+HeaderFilterRegex: '.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/foo.cpp

@@ -0,0 +1,3 @@
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s -- 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK: foo.h:1:12: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/foo.h

@@ -0,0 +1 @@
+struct X { X(int); };

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/subfolder/.clang-tidy

@@ -0,0 +1,2 @@
+InheritParentConfig: true
+HeaderFilterRegex: 'subfolder/.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/subfolder/bar.cpp

@@ -0,0 +1,6 @@
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s -- -I $(dirname %S) 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK-NOT: foo.h:1:12: warning: single-argument constructors must be marked explicit
+
+#include "bar.h"
+// CHECK: bar.h:1:13: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/inheritance/subfolder/bar.h

@@ -0,0 +1 @@
+struct XX { XX(int); };

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/simple/.clang-tidy

@@ -0,0 +1 @@
+HeaderFilterRegex: '.*'

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/simple/foo.cpp

@@ -0,0 +1,3 @@
+// RUN: clang-tidy -checks=-*,google-explicit-constructor %s -- 2>&1 | FileCheck %s
+#include "foo.h"
+// CHECK: foo.h:1:12: warning: single-argument constructors must be marked explicit

clang-tools-extra/test/clang-tidy/infrastructure/header-filter-from-config/simple/foo.h

@@ -0,0 +1 @@
+struct X { X(int); };