This reorders the common and c++ global tree arrays. It introduces a
module-specific High Water Mark, below which are the immutable slots
initialized at startup and beyond which are the lazily filled slots
(and a few immutables we need to locate by name lookup anyway).
gcc/c-family/
* c-common.h (enum c_tree_index): Reorder to place lazy fields
after newly-added CTI_MODULE_HWM.
gcc/cp/
* cp-tree.h (enum cp_tree_index): Reorder to place lazy fields
after newly-added CPTI_MODULE_HWM.