StackGuard when DotNetBuildUseMonoRuntime

[T-Gro]

Add StackGuard for visitSynExpr under Mono runtime + source-build CI for Mono configuration

Problem

The visitSynExpr function in FileContentMapping.fs uses deep recursion through a continuation-passing style visit function. On Mono-based .NET runtimes (used in certain VMR source-build configurations), the default stack size is smaller, which can lead to StackOverflowException when processing deeply nested syntax trees.

Changes

Compiler: StackGuard for Mono runtime (FileContentMapping.fs)

When building with DotNetBuildUseMonoRuntime=true, the recursive visit function in visitSynExpr is replaced with a visitGuarded variant that uses StackGuard to check remaining stack space and spill to a new thread when needed. This is gated behind #if BUILD_USING_MONO so the CoreCLR path remains unchanged.

• Added open FSharp.Compiler.DiagnosticsLogger for StackGuard access.
• Under BUILD_USING_MONO: visitGuarded takes a StackGuard parameter; each recursive visit call goes through sg.Guard(...).
• Under CoreCLR (default): the existing visit function is kept as-is with no overhead.

Project: conditional BUILD_USING_MONO define (FSharp.Compiler.Service.fsproj)

Added <DefineConstants> that sets BUILD_USING_MONO when DotNetBuildUseMonoRuntime is true, matching the convention used by the VMR (dotnet/dotnet) for Mono source-build scenarios.

CI: second source-build platform for Mono configuration (azure-pipelines-PR.yml)

Added a Managed_MonoRuntime source-build platform that passes /p:DotNetBuildUseMonoRuntime=true alongside --source-build. This exercises the Mono code paths in PR CI. The property flows through build.sh -> eng/build.sh -> MSBuild via the existing /p:* pass-through mechanism.

Note: This does not run the build on the Mono runtime itself -- the CI container still uses a CoreCLR-based SDK. The DotNetBuildUseMonoRuntime property enables Mono-specific adaptations (disable R2R, enable StackGuard, set BUILD_USING_MONO define). Actual Mono-runtime execution happens when F# is built inside the VMR with a Mono-based SDK.

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cc @dotnet/fsharp-team-msft