C#: Restrict data flow for multi-bodied methods

[hvitved]

When a method is extracted multiple times, in different files, we end up with a single method containing multiple method bodies. This can sometimes throw data flow off, so this PR does two things:

• Treat calls to multi-bodied methods as virtual dispatch, using the newly introduced DataFlowSecondLevelScope.
• When a call targets a multi-bodied method where one of the bodies is in the same file as the call, prohibit flow to all other bodies, using the newly introduced NodeRegion.

Exampe tuple counts (before)

#	n	stage	nodes	fields	conscand	states	tuples	calledges	tfnodes	tftuples
1	10	1 Fwd	13,187,870	185,706	-1	1	17,590,710	-1	-1	-1
2	15	1 Rev	5,671,136	120,307	-1	1	8,255,144	27,776,216	-1	-1
3	20	2 Fwd	3,165,633	59,851	79,047	1	5,916,802	12,526,882	0	0
4	25	2 Rev	2,189,692	50,677	66,010	1	3,000,179	2,784,088	0	0
5	30	3 Fwd	1,261,133	39,727	3,777,415	1	40,202,863	2,028,817	188,481	1,738,243
6	35	3 Rev	989,110	35,915	3,718,434	1	2,372,076	1,641,461	165,057	2,029,199
7	40	4 Fwd	291,184	13,682	746,048	1	111,372,924	1,058,269	74,870	445,043
8	45	4 Rev	225,919	13,193	653,314	1	20,928,943	1,004,960	37,409	410,968
9	50	5 Fwd	127,287	2,524	19,412	1	1,175,226	872,596	15,696	156,520
10	55	5 Rev	110,916	2,265	12,969	1	439,440	837,375	10,564	164,147
11	60	6 Fwd	109,488	2,243	18,826	1	1,361,311	-1	-1	-1
12	65	6 Rev	57,757	1,451	13,063	1	989,695	-1	-1	-1

After

#	n	stage	nodes	fields	conscand	states	tuples	calledges	tfnodes	tftuples
1	10	1 Fwd	13,187,870	185,706	-1	1	17,590,710	-1	-1	-1
2	15	1 Rev	5,671,136	120,307	-1	1	8,255,144	27,776,214	-1	-1
3	20	2 Fwd	3,163,921	59,833	79,020	1	5,910,484	12,522,597	0	0
4	25	2 Rev	2,186,671	50,628	65,949	1	2,994,764	2,782,439	0	0
5	30	3 Fwd	1,259,837	39,703	3,777,163	1	40,165,525	2,028,199	188,439	1,738,170
6	35	3 Rev	987,593	35,883	3,718,213	1	2,365,799	1,640,881	165,021	2,028,985
7	40	4 Fwd	290,478	13,669	745,225	1	111,249,989	1,058,028	74,838	444,932
8	45	4 Rev	225,341	13,180	653,211	1	20,889,984	1,004,774	37,379	410,886
9	50	5 Fwd	127,095	2,523	19,404	1	1,056,111	872,527	15,690	156,337
10	55	5 Rev	110,796	2,263	12,961	1	438,541	837,334	10,560	164,141
11	60	6 Fwd	109,371	2,241	18,811	1	1,241,825	-1	-1	-1
12	65	6 Rev	57,692	1,450	13,046	1	870,266	-1	-1	-1

[hvitved]

Superseded by #16817.