Remove -incremental when using -whole-module-optimization

Passing both results in noisy remarks when building from the command line like:

```
remark: Incremental compilation has been disabled: it is not compatible with whole module optimization
```

https://forums.swift.org/t/incremental-compilation-has-been-disabled-it-is-not-compatible-with-whole-module-optimization/66092

lib/BuildSystem/BuildSystem.cpp

@@ -2583,8 +2583,7 @@ class SwiftCompilerShellCommand : public ExternalCommand {
       result.push_back("-module-alias");
       result.push_back(nameAndAlias);
     }
-
-    result.push_back("-incremental");
+
     result.push_back("-emit-dependencies");
     if (!moduleOutputPath.empty()) {
       result.push_back("-emit-module");
@@ -2600,6 +2599,8 @@ class SwiftCompilerShellCommand : public ExternalCommand {
       result.push_back("-whole-module-optimization");
       result.push_back("-num-threads");
       result.push_back(numThreads);
+    } else {
+      result.push_back("-incremental");
     }
     result.push_back("-c");
     for (const auto& source: sourcesList) {