Many HotSpot files need instanceKlass.hpp, but only a small number of them directly use the ConstantPool type.

instanceKlass.hpp is large (929 lines) and in turn includes many other header files.

We should remove the inclusion of constantPool.hpp from instanceKlass.hpp to help reduce HotSpot compilation time.

constantPool.hpp is currently included by 809 HotSpot .o files, as measured by https://github.com/iklam/tools/blob/main/headers/whoincludes.tcl . The goal is to eventually reduce this to a much smaller number. This RFE is just an initial step.