CompilationUnitStructureRequestor.resolveDuplicates has poor performance
in extremely large source files with many field definitions. In
particular the class uses Eclipse custom hash maps, which show worse
performance than Java hash maps.

This change switches the two hash maps which show poor performance to
JRE hash maps.

Change-Id: I084232a59c8330769081bd7bdbaa8a3d5e35d5b9
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>