[WIP] [RFC FS-1060] Nullness checking (applied to codebase) (#15265)

* enablement

* enablement

* fix build

* fix build

* fantomas

* selective adoption

Author: dsyme

src/Compiler/FSharp.Compiler.Service.fsproj

@@ -13,6 +13,7 @@
     <AssemblyName>FSharp.Compiler.Service</AssemblyName>
     <AllowCrossTargeting>true</AllowCrossTargeting>
     <DefineConstants>$(DefineConstants);COMPILER</DefineConstants>
+    <CheckNulls>true</CheckNulls>
     <DefineConstants Condition="'$(FSHARPCORE_USE_PACKAGE)' == 'true'">$(DefineConstants);FSHARPCORE_USE_PACKAGE</DefineConstants>
     <OtherFlags>$(OtherFlags) --extraoptimizationloops:1</OtherFlags>
     <!-- 1182: Unused variables -->

src/FSharp.Build/FSharp.Build.fsproj

@@ -9,6 +9,7 @@
     <AssemblyName>FSharp.Build</AssemblyName>
     <NoWarn>$(NoWarn);75</NoWarn> <!-- InternalCommandLineOption -->
     <AllowCrossTargeting>true</AllowCrossTargeting>
+    <CheckNulls>true</CheckNulls>
     <DefineConstants>$(DefineConstants);LOCALIZATION_FSBUILD</DefineConstants>
     <NoWarn>$(NoWarn);NU1701;FS0075</NoWarn>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>