analyzer: only call get_diagnostic_tree when it's needed

analyzer: only call get_diagnostic_tree when it's needed

impl_sm_context::get_diagnostic_tree could be expensive, and
I find myself needing to put a breakpoint on it to debug
PR analyzer/99771, so only call it if we're about to use
the result.

gcc/analyzer/ChangeLog:
	* sm-file.cc (fileptr_state_machine::on_stmt): Only call
	get_diagnostic_tree if the result will be used.
	* sm-malloc.cc (malloc_state_machine::on_stmt): Likewise.
	(malloc_state_machine::on_deallocator_call): Likewise.
	(malloc_state_machine::on_realloc_call): Likewise.
	(malloc_state_machine::on_realloc_call): Likewise.
	* sm-sensitive.cc
	(sensitive_state_machine::warn_for_any_exposure): Likewise.
	* sm-taint.cc (taint_state_machine::on_stmt): Likewise.