cmSourceFile: Check if a file is GENERATED first in the full path computation

In `cmSourceFile::FindFullPath` check first if the file is GENERATED before
aborting on `FindFullPathFailed`.  This allows recomputation of the full path
when the GENERATED property was set after the file path was computed with an
error.

Source/cmSourceFile.cxx

@@ -113,11 +113,6 @@ std::string const& cmSourceFile::GetFullPath() const
 
 bool cmSourceFile::FindFullPath(std::string* error)
 {
-  // If this method has already failed once do not try again.
-  if (this->FindFullPathFailed) {
-    return false;
-  }
-
   // If the file is generated compute the location without checking on disk.
   if (this->GetIsGenerated()) {
     // The file is either already a full path or is relative to the
@@ -127,6 +122,11 @@ bool cmSourceFile::FindFullPath(std::string* error)
     return true;
   }
 
+  // If this method has already failed once do not try again.
+  if (this->FindFullPathFailed) {
+    return false;
+  }
+
   // The file is not generated.  It must exist on disk.
   cmMakefile const* makefile = this->Location.GetMakefile();
   // Location path