Fix -fmodule-map-file pointing to non-existent path

We were passing this unconditionally which would break in the case where we weren't generated a module map because an explicit one exists.

rdar://74115399

Author: neonichu

Sources/XCBuildSupport/PIFBuilder.swift

@@ -583,6 +583,9 @@ final class PackagePIFProjectBuilder: PIFProjectBuilder {
                         export *
                     }
                     """
+
+                // Pass the path of the module map up to all direct and indirect clients.
+                impartedSettings[.OTHER_CFLAGS, default: ["$(inherited)"]].append("-fmodule-map-file=\(moduleMapFile)")
             } else {
                 moduleMapFileContents = nil
             }
@@ -607,8 +610,6 @@ final class PackagePIFProjectBuilder: PIFProjectBuilder {
             settings[.MODULEMAP_FILE_CONTENTS] = moduleMapFileContents
         }
 
-        // Pass the path of the module map up to all direct and indirect clients.
-        impartedSettings[.OTHER_CFLAGS, default: ["$(inherited)"]].append("-fmodule-map-file=\(moduleMapFile)")
         impartedSettings[.OTHER_LDRFLAGS] = []
 
         if target.underlyingTarget.isCxx {