Start building a universal System.Linq.Expressions library (#61952)

System.Linq.Expressions currently offers multiple build time definitions to build different flavors of the library:

* `FEATURE_COMPILE` (defined everywhere except iOS/tvOS/Catalyst, and NativeAOT) - enables Reflection.Emit-based expression tree compiler.
* `FEATURE_INTERPRET` (always defined and not actually possible to build without) - enables the expression interpreter
* `FEATURE_DLG_INVOKE` (defined everywhere except NativeAOT, but we likely need to be able to run without it on iOS too because there's uninvestigated bugs around it ActiveIssue'd in #54970) - in the interpreter, use a delegate to invoke `CallInstructions` instead of `MethodInfo.Invoke`. The delegate might have to be Reflection.Emitted if it's not supportable with `Func`/`Action` (that's the uninvestigated iOS/tvOS/Catalyst bug).

For #61231, we need to be able to build a single System.Linq.Expression library that can switch between these build-time configurations _at publish time_ since we don't want to build a separate S.L.Expressions library for NativeAOT. There are advantages in having this setup for non-NativeAOT scenarios too. This pull request accomplishes that by mechanically changing the `#define`s into feature switches.

The feature switch is placed in the last proposed location for https://github.com/dotnet/runtime/issues/17973. I expect we'll want such API to be public at some point; now that Xamarin and NativeAOT use this formerly .NET Native-only thing the API request became relevant again.

This pull request is focused on the mechanical replacement of `#defines` with feature switches and it's already a lot bigger than I'm comfortable with.

There's some obvious "`!FEATURE_COMPILE` means this is .NET Native with everything what that meant" that I did not touch because this is meant to be a mechanical change. Some cleanup will be needed at some point. Right now this just mostly means we're running fewer tests than we could.

Validation:
* Verified that we're still running the same number of tests with CoreCLR as we previously were and they're all passing.
* Verified we're getting mostly the same size of the S.L.Expressions library on iOS (433 kB grew to 436 kB, the diffs are expected).
* Verified things work on the NativeAOT side as well.

1 files changed, 2 insertions, 0 deletions

diff --git a/eng/illink.targets b/eng/illink.targets
index 6c2d95c8477..018075d8b66 100644
--- a/eng/illink.targets
+++ b/eng/illink.targets
@@ -236,6 +236,7 @@
       <ILLinkArgs Condition="'$(ILLinkRewritePDBs)' == 'true' and Exists('$(ILLinkTrimAssemblySymbols)')">$(ILLinkArgs) -b true</ILLinkArgs>
       <!-- pass the non-embedded descriptors xml file on the command line -->
       <ILLinkArgs Condition="'$(ILLinkDescriptorsLibraryBuildXml)' != ''">$(ILLinkArgs) -x "$(ILLinkDescriptorsLibraryBuildXml)"</ILLinkArgs>
+      <ILLinkArgs Condition="'$(ILLinkSubstitutionsLibraryBuildXml)' != ''">$(ILLinkArgs) --substitutions "$(ILLinkSubstitutionsLibraryBuildXml)"</ILLinkArgs>
       <!-- suppress warnings with the following codes:
            IL2008: Could not find type A specified in resource B
            IL2009: Could not find method A in type B specified in resource C
@@ -254,6 +255,7 @@
       <!-- IL2067-IL2091: Unsatisfied DynamicallyAccessedMembers requirements -->
       <LinkerNoWarn>$(LinkerNoWarn);IL2067;IL2068;IL2069;IL2070;IL2071;IL2072;IL2073;IL2074;IL2075;IL2076;IL2077;IL2078;IL2079;IL2080;IL2081;IL2082;IL2083;IL2084;IL2085;IL2086;IL2087;IL2088;IL2089;IL2090;IL2091</LinkerNoWarn>
       <ILLinkArgs>$(ILLinkArgs) --nowarn $(LinkerNoWarn)</ILLinkArgs>
+      <ILLinkArgs Condition="'$(ILLinkDisableIPConstProp)' == 'true'">$(ILLinkArgs) --disable-opt ipconstprop</ILLinkArgs>
     </PropertyGroup>
 
     <MakeDir Directories="$(ILLinkTrimInputPath)" />