[clang-tools-extra] [clangd] [C++ Modules] Skip PCH when TU imports modules (PR #187432)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

> > Most of your arguments are about limited resources and solution simplicity, but we should focus on quality https://llvm.org/docs/DeveloperPolicy.html#quality
> > Here we have performance regression in clangd (but we should not).
> 
> This is controversial with the following suggestion to disable preamble PCH completely. Again the support of modules in clangd right now is experimental.

If we decide to go this way, we can provide one more possible value for `-pch-storage` command line option (e.g. "none"). So, if a user wants more stable modules support at any cost, he can add `-pch-storage=none` (or maybe this can be set to "none" if we enable experimental C++20 modules support and `-pch-storage` was not set explicitly?). So, in this case we can say that this is a user decision.
Also, the ability to disable preamble optimization can be useful in some other scenarios, including debugging hard to reproduce problem or maybe in read-only modes.


https://github.com/llvm/llvm-project/pull/187432