[clang-tidy][readability-identifier-naming] Resolve symlinks for checking style for file (#81985)

Summary:
Some build systems create symlinks in a temporary build directory for
headers in the source tree for isolation purposes. These symlinks
prevent `readability-identifier-naming` detecting issues and applying
fixes. Without this fix clang-tidy is checking .clang-tidy config file
in a temporary directory instead of source source location.

Test Plan: check-clang-tools

Author: dmpolukhin

clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp

@@ -1408,13 +1408,16 @@ const IdentifierNamingCheck::FileStyle &
 IdentifierNamingCheck::getStyleForFile(StringRef FileName) const {
   if (!GetConfigPerFile)
     return *MainFileStyle;
-  StringRef Parent = llvm::sys::path::parent_path(FileName);
+
+  SmallString<128> RealFileName;
+  llvm::sys::fs::real_path(FileName, RealFileName);
+  StringRef Parent = llvm::sys::path::parent_path(RealFileName);
   auto Iter = NamingStylesCache.find(Parent);
   if (Iter != NamingStylesCache.end())
     return Iter->getValue();
 
   llvm::StringRef CheckName = getID();
-  ClangTidyOptions Options = Context->getOptionsForFile(FileName);
+  ClangTidyOptions Options = Context->getOptionsForFile(RealFileName);
   if (Options.Checks && GlobList(*Options.Checks).contains(CheckName)) {
     auto It = NamingStylesCache.try_emplace(
         Parent,

clang-tools-extra/docs/ReleaseNotes.rst

@@ -188,6 +188,10 @@ Changes in existing checks
   <clang-tidy/checks/readability/redundant-inline-specifier>` check to properly
   emit warnings for static data member with an in-class initializer.
 
+- Improved :doc:`readability-identifier-naming
+  <clang-tidy/checks/readability/identifier-naming>` check in `GetConfigPerFile`
+  mode by resolving symbolic links to header files.
+
 Removed checks
 ^^^^^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/symlink/include/.clang-tidy

@@ -0,0 +1,4 @@
+Checks: readability-identifier-naming
+CheckOptions:
+  readability-identifier-naming.GlobalConstantCase: CamelCase
+  readability-identifier-naming.GlobalConstantPrefix: k

clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/symlink/include/test.h

@@ -0,0 +1 @@
+const int global_const = 5;

clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-symlink.cpp

@@ -0,0 +1,18 @@
+// Specify `-std=c++20` to run test only once becuase test expects changes
+// in the header file so it fails if runs multiple times with different
+// `-std` flags as check_clang_tidy doesn by default.
+//
+// RUN: rm -rf %T/symlink
+// RUN: cp -r %S/Inputs/identifier-naming/symlink %T/symlink
+// RUN: mkdir -p %T/symlink/build
+// RUN: ln -s %T/symlink/include/test.h %T/symlink/build/test.h
+// RUN: %check_clang_tidy -std=c++20 %s readability-identifier-naming %t -- --header-filter="test.h" --config-file=%S/Inputs/identifier-naming/symlink/include/.clang-tidy -- -I %T/symlink/build
+// UNSUPPORTED: system-windows
+
+#include "test.h"
+
+int foo() {
+    return global_const;
+    // CHECK-MESSAGES: warning: invalid case style for global constant 'global_const' [readability-identifier-naming]
+    // CHECK-FIXES: return kGlobalConst;
+}