[clang] [clang][lex] Use `FileManager` to make prebuilt module paths absolute (PR #165347)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/165347

This PR switches from using `llvm::sys::fs::make_absolute()` to `FileManager::makeAbsolutePath()` so that `FileSystemOptions` (i.e. the `-working-directory` option) and the `VFS`'s CWD have a say in how the prebuilt module paths are resolved. This matches how the rest of the compiler treats input files.

>From b9d809e1bde57c91607333a24e236d918b052b87 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Mon, 27 Oct 2025 20:13:49 -0700
Subject: [PATCH] [clang][lex] Use `FileManager` to make prebuilt module paths
 absolute

---
 clang/lib/Lex/HeaderSearch.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index 65c324c10ca5d..f05c28fd7a123 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -221,7 +221,7 @@ std::string HeaderSearch::getPrebuiltModuleFileName(StringRef ModuleName,
   // file.
   for (const std::string &Dir : HSOpts.PrebuiltModulePaths) {
     SmallString<256> Result(Dir);
-    llvm::sys::fs::make_absolute(Result);
+    FileMgr.makeAbsolutePath(Result);
     if (ModuleName.contains(':'))
       // The separator of C++20 modules partitions (':') is not good for file
       // systems, here clang and gcc choose '-' by default since it is not a
@@ -246,7 +246,7 @@ std::string HeaderSearch::getPrebuiltImplicitModuleFileName(Module *Module) {
   StringRef ModuleCacheHash = HSOpts.DisableModuleHash ? "" : getModuleHash();
   for (const std::string &Dir : HSOpts.PrebuiltModulePaths) {
     SmallString<256> CachePath(Dir);
-    llvm::sys::fs::make_absolute(CachePath);
+    FileMgr.makeAbsolutePath(CachePath);
     llvm::sys::path::append(CachePath, ModuleCacheHash);
     std::string FileName =
         getCachedModuleFileNameImpl(ModuleName, ModuleMapPath, CachePath);