SwiftPM complains about @main in non main.swift file when integrated driver is not enabled

One of several different code paths in BuildPlan.swift are taken depending on whether the integrated Swift driver is enabled, whether explicit modules are enabled, etc.  In the non-integrated-driver case, the code path for adding a `-parse-as-library` when necessary wasn't taken.  This moves that code to a more central place.  A unit test is needed that will make sure the flag is added in this case as well.

rdar://78080245

Author: abertelrud

Sources/Build/BuildPlan.swift

@@ -757,6 +757,11 @@ public final class SwiftTargetBuildDescription {
                 }
             }
         }
+        
+        // If the target needs to be parsed without any special semantics involving "main.swift", do so now.
+        if self.needsToBeParsedAsLibrary {
+            args += ["-parse-as-library"]
+        }
 
         // Only add the build path to the framework search path if there are binary frameworks to link against.
         if !libraryBinaryPaths.isEmpty {
@@ -809,10 +814,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         if buildParameters.useWholeModuleOptimization {
             result.append("-whole-module-optimization")
             result.append("-num-threads")
@@ -849,10 +850,6 @@ public final class SwiftTargetBuildDescription {
         result.append("-experimental-skip-non-inlinable-function-bodies")
         result.append("-force-single-frontend-invocation")
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
-
         // FIXME: Handle WMO
 
         for source in target.sources.paths {
@@ -896,9 +893,6 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if self.needsToBeParsedAsLibrary {
-            result.append("-parse-as-library")
-        }
         // FIXME: Handle WMO
 
         result.append("-c")