[clang-tools-extra] [clangd] [Modules] Use ASTReader directly in IsModuleFileUpToDate (PR #113879)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

>  if you think that's fine, i think we can always create that astreader directly here, without a compilerinstance as well.

I like the idea more.  (so I typed 'updated too quickly') The most important reason I did the previous change is, I think the compiler invocation and compiler instance is not needed to check if source files recorded in BMI are out dated or not. I feel  they are redundant and want to get rid of that. And now you pointed out that, actually ASTUnit may be redundant and what we need actually is the ASTReader only. I like the idea. (Although it introduces the codes a little bit longer)

https://github.com/llvm/llvm-project/pull/113879