Initial support for binding patterns in SemIR (#4221)

Introduces the `BindingPattern` and `SymbolicBindingPattern` insts, and
a separate stack of pattern blocks that they are emitted into. The
intent is to generate the corresponding pattern-matching insts (like
`BindName`) from them in a separate pass, but that is deferred to future
PRs.

See
[here](https://docs.google.com/document/d/1U_vQH17V893J9aF1LJXUnFYBNSs2MjKl4bJPaWCB2zo/edit?usp=sharing&resourcekey=0-w0xGYZ0An31Kpz-wvzSXwQ)
for the design this is based on, but note that during review we have
chosen to deviate from that design by putting the patterns in separate
blocks, and omitting the "forward references" from a `BindingPattern` to
its corresponding `BindName`. This in turn necessitates having separate
inst kinds for symbolic and non-symbolic binding patterns.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_binding_pattern.cpp

@@ -167,6 +167,20 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
       // TODO: Bindings should come into scope immediately in other contexts
       // too.
       context.AddNameToLookup(name_id, bind_id);
+      auto entity_name_id =
+          context.insts().GetAs<SemIR::AnyBindName>(bind_id).entity_name_id;
+      if (is_generic) {
+        context.AddPatternInst<SemIR::SymbolicBindingPattern>(
+            name_node,
+            {.type_id = cast_type_id, .entity_name_id = entity_name_id});
+      } else {
+        context.AddPatternInst<SemIR::BindingPattern>(
+            name_node,
+            {.type_id = cast_type_id, .entity_name_id = entity_name_id});
+      }
+      // TODO: use the pattern insts to generate the pattern-match insts
+      // at the end of the full pattern, instead of eagerly generating them
+      // here.
       break;
     }