[clang-tools-extra] [wip][clangd] Introduce --skip-preamble-build command line option (PR #189284)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> I failed to understand what you want to say. If we change the file by adding a new #include, of course, it should change. But I guess you're talking about something different?

If we have headers from your example

And one more extra

Test.h
```
import m;
```

main.cpp is
```
#include "Dummy.h"

int dummy; // we cut preamble here

#include "Test.h"

int a = mm; 
```

If this works correctly without this patch, then 


```
#include "Test.h"

int a = mm; 
```
also should be ok with `--skip-preamble-build=true`

For me at `vvv` modifications, hover at `mm` shows correct modified value.

https://github.com/llvm/llvm-project/pull/189284