Error while trying to build SourceKit-LSP on Linux

[akyrtzi (Argyrios Kyrtzidis)]

Fixed a bunch of issues related to getting the service to work properly on linux:

[SKCore] If 'Platform.currentPlatform' is nil use 'so' as library extension

[IndexStoreCXX.h] Fix the crashing issue with using the '*_f' functions

[CIndexStoreDB] Fix use after free issue with temporary strings