[clang] [clang][modules] Dependency Scanning: Temporarily Turn Off Negative Directory Caching (PR #134698)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

To be clear, this PR is basically cherry-picking https://github.com/swiftlang/llvm-project/pull/9076, but we are only taking what is necessary to fix our problem at the moment. I updated the description to reflect this. 

> What does temporarily mean here? Is it until the project identified with errors can resolve themselves? Is it until error detection in the build system can catch cases where negative stating can cause incorrect build errors? Is it a pre-determined time frame?

My intention is that this should be turned back on when we know the root cause of the build failures, and have some ways to fix the failures. While I don't have an exact date to turn this back on, I expect some updates this week. Ideally, I hope we can turn the optimization back on here in a week or two. 

> I'm concerned about disabling this optimization entirely without an understanding of how it will be turned back on. What is the added performance cost of doing this in environments where I/O accesses can be expensive?

@jansvoboda11 may I get your comments on the possible performance impact? I suspect this is tolerable for a few weeks because we have released compilers with https://github.com/swiftlang/llvm-project/pull/9076 in them. 

https://github.com/llvm/llvm-project/pull/134698