Turn 'bool force_collect' parameter to 'ggc_collect' into an 'enum ggc_collect mode'

Turn 'bool force_collect' parameter to 'ggc_collect' into an 'enum ggc_collect mode'

... to make the meaning more explicit to the reader of the code.

Follow-up to recent commit 0edf2e81bb02cba43b649b3f6e7258b68a779ac0
"Turn global 'ggc_force_collect' variable into 'force_collect' parameter to
'ggc_collect'".

	gcc/
	* ggc.h (enum ggc_collect): New.
	(ggc_collect): Use it.
	* ggc-page.c: Adjust.
	* ggc-common.c: Likewise.
	* ggc-tests.c: Likewise.
	* read-rtl-function.c: Likewise.
	* selftest-run-tests.c: Likewise.
	* doc/gty.texi (Invoking the garbage collector): Likewise.

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