[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

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());
 }
 

clang/test/Modules/implicit-module-with-missing-path.cpp

@@ -0,0 +1,12 @@
+// This tests that the compiler wouldn't crash if the module path misses
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t/subdir
+// RUN: echo "export module C;" >> %t/subdir/C.cppm
+// RUN: echo -e "export module B;\nimport C;" >> %t/B.cppm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/subdir/C.cppm -o %t/subdir/C.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t/subdir %t/B.cppm -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %s -fsyntax-only -verify
+
+import B;
+import C; // expected-error {{module 'C' is needed but has not been provided, and implicit use of module files is disabled}}