Permit explicit tags with '_' switch prong

Mainly affects ZIR representation of switch_block[_ref]
and special prong (detection) logic for switch.
Adds a new SpecialProng tag 'absorbing_under' that allows
specifying additional explicit tags in a '_' prong which
are respected when checking that every value is handled
during semantic analysis but are not transformed into AIR
and instead 'absorbed' by the '_' branch.

test/cases/compile_errors/switch_expression-non_exhaustive_absorbing.zig

@@ -0,0 +1,33 @@
+const E = enum(u8) {
+    a,
+    b,
+    _,
+};
+const U = union(E) {
+    a: i32,
+    b: u32,
+};
+pub export fn entry1() void {
+    const e: E = .b;
+    switch (e) { // error: switch not handling the tag `b`
+        .a, _ => {},
+    }
+}
+pub export fn entry2() void {
+    const u = U{ .a = 2 };
+    switch (u) { // error: `_` prong not allowed when switching on tagged union
+        .a => {},
+        .b, _ => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :12:5: error: switch must handle all possibilities
+// :3:5: note: unhandled enumeration value: 'b'
+// :1:11: note: enum 'tmp.E' declared here
+// :18:5: error: '_' prong only allowed when switching on non-exhaustive enums
+// :20:13: note: '_' prong here
+// :18:5: note: consider using 'else'

test/cases/compile_errors/switching_with_exhaustive_enum_has___prong_.zig

@@ -16,5 +16,5 @@ pub export fn entry() void {
 // target=native
 //
 // :7:5: error: '_' prong only allowed when switching on non-exhaustive enums
-// :10:11: note: '_' prong here
+// :10:9: note: '_' prong here
 // :7:5: note: consider using 'else'

test/cases/compile_errors/switching_with_non-exhaustive_enums.zig

@@ -39,5 +39,5 @@ pub export fn entry3() void {
 // :1:11: note: enum 'tmp.E' declared here
 // :19:5: error: switch on non-exhaustive enum must include 'else' or '_' prong
 // :26:5: error: '_' prong only allowed when switching on non-exhaustive enums
-// :29:11: note: '_' prong here
+// :29:9: note: '_' prong here
 // :26:5: note: consider using 'else'