Close https://github.com/llvm/llvm-project/issues/62707

As we discussed before, we'll forbid the use of implicit generated path
for C++20 modules. And as I mentioned in
https://github.com/llvm/llvm-project/issues/62707, we've emitted a
warning for clang17 and we'll make it a hard error in clang18. And the
patch addresses the decision.

A step to address https://github.com/llvm/llvm-project/issues/62707.

It is not user friendly enough to drop the implicitly generated path
directly. Let's emit the warning first and drop it in the next version.

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