[clang-tools-extra] [clangd] Implement simple folding of preprocessor branches (PR #80592)

[Ruihua Dong via cfe-commits]

144026 wrote:

The build pipeline failure is on Windows x64, and appears to be some random error irrelevant to this patch:
```
MT: command "C:\Program Files (x86)\Windows Kits\10\bin\10.0.19041.0\x64\mt.exe /nologo /manifest bin\dexp.exe.manifest /outputresource:bin\dexp.exe;#1" failed (exit code 0x1f) with the following output:
mt.exe : general error c101008d: Failed to write the updated manifest to the resource of file "bin\dexp.exe". Operation did not complete successfully because the file contains a virus or potentially unwanted software.
```


https://github.com/llvm/llvm-project/pull/80592