An option to always emit attribute symbols in Symbols API

[auduchinok]

We inspect symbol attributes when looking for tests or doing some analysis. Some attributes we're particularly interested in are erased in compiled code due to use of ConditionalAttribute and these attributes aren't emitted in Symbols API either.
Example of such attributes include NotNull, CanBeNull, UsedImplicitly from JetBrains.Annotations package. Ignoring the use of ConditionalAttribute in TypeChecker.fs helps in our case, makes it emit these attributes.

I'd like to add an option to always emit such attributes by ignoring condition. Would FSharpProjectOptions be a suitable place for it? It would require to pass a bool to TcConfigBuilder, TcConfig and then through type check calls.
Another option is to make it always emit such attributes by adding COMPILER_PUBLIC_API condition directive to the code below so all attributes would be available for analysis by clients. Also, another property for checking whether an attribute is actually emitted may be added to FSharpAttribute type.
https://github.com/Microsoft/visualfsharp/blob/fefae464c7fb4f15d5b0a19b2e8bfff123a692ca/src/fsharp/TypeChecker.fs#L10528-L10532

[dsyme]

@auduchinok I'm fine with you adding this. A hidden command-line flag may be easiest to reduce plumbing, though I'm don't mind too much either way

It's possible some other things should come under this, i.e. the flag should be a general "view the elaborated source before erasure" flag

[auduchinok]

@dsyme Could you advice a name suitable for the general flag, please?

[abelbraaksma]

Doesn't this cause problems with the CLI? I remember that the ConditionalAttribute is required to result in erasing an entire method (provided the condition is false), meaning it can't even appear in the assembly. If that requirement is still valid, doesn't it conflict with your solution?

[auduchinok]

@abelbraaksma It makes any difference only when analysing code in FCS, not when compiling code.