[C++20] [Modules] Check if modulemap exists to avoid crash in implicit used C++ module

An impilt used of C++ module without prebuild path may cause crash.

For example:

```
// ./dir1/C.cppm
export module C;
// ./dir2/B.cppm
export module B;
import C;
// ./A.cpp
import B;
import C;
```

When we compile A.cpp without the prebuild path of C.pcm, the compiler
will crash.

```
clang++ -std=c++20 --precompile -c ./dir1/C.cppm -o ./dir1/C.pcm
clang++ -std=c++20 --precompile -fprebuilt-module-path=./dir2  -c
./dir2/B.cppm -o ./dir2/B.pcm
clang++ -std=c++20 -fprebuilt-module-path=./dir2 A.cpp

```

The prebuilt path of module C is cached when import module B, and in the
function HeaderSearch::getCachedModuleFileName, the compiler try to get
the filename by modulemap without check if modulemap exists, and there
is no modulemap in C++ module.

Reviewed By: ChuanqiXu

Differential review: https://reviews.llvm.org/D119426

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index 19e284f..500f569 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -172,6 +172,10 @@ void HeaderSearch::getHeaderMapFileNames(
 std::string HeaderSearch::getCachedModuleFileName(Module *Module) {
   const FileEntry *ModuleMap =
       getModuleMap().getModuleMapFileForUniquing(Module);
+  // The ModuleMap maybe a nullptr, when we load a cached C++ module without
+  // *.modulemap file. In this case, just return an empty string.
+  if (ModuleMap == nullptr)
+    return {};
   return getCachedModuleFileName(Module->Name, ModuleMap->getName());
 }