double checked locking for AdditionalTextFile.GetText (#41383)
double checked locking for AdditionalTextFile.GetText (#41383)

* Add double checked locking for AdditionalTextFile.GetText.

* Use LazyInitializer

* Use Lazy instead of LazyInitializer

* Make _text readonly

physhi/roslyn,wvdd007/roslyn,heejaechang/roslyn,ErikSchierboom/roslyn,tannergooding/roslyn,KirillOsenkov/roslyn,weltkante/roslyn,weltkante/roslyn,sharwell/roslyn,agocke/roslyn,tmat/roslyn,physhi/roslyn,jasonmalinowski/roslyn,agocke/roslyn,davkean/roslyn,gafter/roslyn,gafter/roslyn,KevinRansom/roslyn,dotnet/roslyn,sharwell/roslyn,AlekseyTs/roslyn,aelij/roslyn,jasonmalinowski/roslyn,eriawan/roslyn,brettfo/roslyn,abock/roslyn,panopticoncentral/roslyn,genlu/roslyn,sharwell/roslyn,reaction1989/roslyn,tmat/roslyn,KirillOsenkov/roslyn,CyrusNajmabadi/roslyn,physhi/roslyn,mavasani/roslyn,panopticoncentral/roslyn,mgoertz-msft/roslyn,brettfo/roslyn,diryboy/roslyn,eriawan/roslyn,wvdd007/roslyn,diryboy/roslyn,dotnet/roslyn,reaction1989/roslyn,davkean/roslyn,jmarolf/roslyn,AmadeusW/roslyn,shyamnamboodiripad/roslyn,tannergooding/roslyn,stephentoub/roslyn,brettfo/roslyn,mgoertz-msft/roslyn,AmadeusW/roslyn,diryboy/roslyn,tmat/roslyn,wvdd007/roslyn,reaction1989/roslyn,tannergooding/roslyn,stephentoub/roslyn,dotnet/roslyn,ErikSchierboom/roslyn,CyrusNajmabadi/roslyn,genlu/roslyn,KevinRansom/roslyn,heejaechang/roslyn,KevinRansom/roslyn,jmarolf/roslyn,bartdesmet/roslyn,shyamnamboodiripad/roslyn,mavasani/roslyn,jmarolf/roslyn,aelij/roslyn,AlekseyTs/roslyn,CyrusNajmabadi/roslyn,davkean/roslyn,ErikSchierboom/roslyn,stephentoub/roslyn,bartdesmet/roslyn,mavasani/roslyn,heejaechang/roslyn,eriawan/roslyn,abock/roslyn,agocke/roslyn,bartdesmet/roslyn,AmadeusW/roslyn,mgoertz-msft/roslyn,AlekseyTs/roslyn,abock/roslyn,aelij/roslyn,panopticoncentral/roslyn,jasonmalinowski/roslyn,KirillOsenkov/roslyn,genlu/roslyn,shyamnamboodiripad/roslyn,gafter/roslyn,weltkante/roslyn