[C++20] [Modules] Allow to compile a pcm with and without -fPIC

seperately

We can compile a module unit in 2 phase compilaton:

```
clang++ -std=c++20 a.cppm --precompile -o a.pcm
clang++ -std=c++20 a.pcm -c -o a.o
```

And it is a general requirement that we need to compile a translation
unit with and without -fPIC for static and shared libraries.

But for C++20 modules with 2 phase compilation, it may be waste of time
to compile them 2 times completely. It may be fine to generate one BMI
and compile it with and without -fPIC seperately.

e.g.,

```
clang++ -std=c++20 a.cppm --precompile -o a.pcm
clang++ -std=c++20 a.pcm -c -o a.o
clang++ -std=c++20 a.pcm -c -fPIC -o a-PIC.o
```

Then we can save the time to parse a.cppm repeatedly.