analyzer: add caching to globals with initializers [PR110112]

PR analyzer/110112 notes that -fanalyzer is extremely slow on a source
file with large read-only static arrays, repeatedly building the
same compound_svalue representing the full initializer, and repeatedly
building svalues representing parts of the the full initialiazer.

This patch adds caches for both of these; together they reduce the time
taken by -fanalyzer -O2 on the testcase in the bug for an optimized
build:
  91.2s : no caches (status quo)
  32.4s : cache in decl_region::get_svalue_for_constructor
   3.7s : cache in region::get_initial_value_at_main
   3.1s : both caches (this patch)

gcc/analyzer/ChangeLog:
	PR analyzer/110112
	* region-model.cc (region_model::get_initial_value_for_global):
	Move code to region::calc_initial_value_at_main.
	* region.cc (region::get_initial_value_at_main): New function.
	(region::calc_initial_value_at_main): New function, based on code
	in region_model::get_initial_value_for_global.
	(region::region): Initialize m_cached_init_sval_at_main.
	(decl_region::get_svalue_for_constructor): Add a cache, splitting
	out body to...
	(decl_region::calc_svalue_for_constructor): ...this new function.
	* region.h (region::get_initial_value_at_main): New decl.
	(region::calc_initial_value_at_main): New decl.
	(region::m_cached_init_sval_at_main): New field.
	(decl_region::decl_region): Initialize m_ctor_svalue.
	(decl_region::calc_svalue_for_constructor): New decl.
	(decl_region::m_ctor_svalue): New field.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>