Stale ASCompletion FileCache may cause crashes

[gene-pavlovsky]

Related to #2710.
FD is still crashing on me.

Steps to reproduce:

• Open the test project (see below)
• Open App.hx
• Line 21 - click on HeaderField and press F4. The Header.hx from tink_http library opens, then FD crashes with stack overflow. This happens to me when running under VS debugger. I haven't tried without the debugger.

Test project: https://ufile.io/rczcn
FD version: 49eef1c
Haxe version: d477e6a (a recent nightly newer than 4.0.0-rc.1)
Note: using lix (haxeshim) for managing libraries and haxe versions
Setup:

• npm install lix -g
• cd fd-issue-1
• lix download
• FD Settings: HaxeContext -> Installed Haxe SDKs -> Add > Path: %AppData%\npm
  The SDK should be detected by FD as "Haxe Shim" (version doesn't matter)
• Make sure the project uses this SDK
• Restart FD just in case

[gene-pavlovsky]

So in my case, deleting Data/ASCompletion/FileCache had cured the issue.
Maybe FD should clear the cache under some conditions (any FD update, or particular updates involving changes to the completion-related code)?