Avoid lock when setting project entry point

[Erarndt]

Context

There's a good amount of contention inside of BuildManager.PendBuildRequest() coming from the lock in the body:

    private BuildSubmissionBase<TRequestData, TResultData> PendBuildRequest<TRequestData, TResultData>(
        TRequestData requestData)
        where TRequestData : BuildRequestData<TRequestData, TResultData>
        where TResultData : BuildResultBase
    {
        lock (_syncLock)
        {
            ErrorUtilities.VerifyThrowArgumentNull(requestData);
            ErrorIfState(BuildManagerState.WaitingForBuildToComplete, "WaitingForEndOfBuild");
            ErrorIfState(BuildManagerState.Idle, "NoBuildInProgress");
            VerifyStateInternal(BuildManagerState.Building);

            var newSubmission = requestData.CreateSubmission(this, GetNextSubmissionId(), requestData,
                _buildParameters!.LegacyThreadingSemantics);

            if (_buildTelemetry != null)
            {
                // Project graph can have multiple entry points, for purposes of identifying event for same build project,
                // we believe that including only one entry point will provide enough precision.
                _buildTelemetry.ProjectPath ??= requestData.EntryProjectsFullPath.FirstOrDefault();
                _buildTelemetry.BuildTarget ??= string.Join(",", requestData.TargetNames);
            }

            _buildSubmissions.Add(newSubmission.SubmissionId, newSubmission);
            _noActiveSubmissionsEvent!.Reset();
            return newSubmission;
        }
    }

There's a small amount of CPU time caused by hitting the lock too.

Looks like this can be switched over to use some low-lock patterns to mitigate this. The results after the changes show the contention is essentially eliminated

Changes Made

Testing

Notes