Support for parsing expression patterns (#6977)

When parsing a pattern, if we encounter something that isn't pattern
syntax, try parsing as an expression instead. We only need one-token
lookahead to distinguish pattern syntax from expression syntax.

Track a precedence group through pattern parsing so that we can allow
different kinds of expressions in a top-level pattern (such as the
operand of `let`) and in a nested pattern (such as a subpattern of a
tuple pattern or within grouping parens). For example, we do not allow
`case if ...`, and for now I've chosen to also not allow logical or
relational operators at the top level of a pattern, so `case 1 + 1` is
OK, but `case 1 == 1` and `case true and false` require parentheses.
This decision should be ratified or revisited by a design proposal.

Very basic check support is also provided, only sufficient to form an
`ExprPattern` instruction and nothing beyond that. For now, all pattern
matching against an `ExprPattern` fails with a TODO error. To support
that, I've switched from calling `BeginSubpattern` in the parent handler
of a pattern and `EndSubpatternAs*` in the pattern handler itself to
calling both functions in parent handlers, with `EndSubpattern`
converting an expression into an expression pattern where needed.

Depends on #6976.

Assisted-by: Gemini via Google Antigravity

toolchain/check/handle_binding_pattern.cpp

@@ -155,7 +155,7 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
   }
 
   SemIR::ExprRegionId type_expr_region_id =
-      EndSubpatternAsExpr(context, type_expr.inst_id);
+      ConsumeSubpatternExpr(context, type_expr.inst_id);
 
   // The name in a generic binding may be wrapped in `template`.
   bool is_generic = node_kind == Parse::NodeKind::CompileTimeBindingPattern;
@@ -453,7 +453,9 @@ auto HandleParseNode(Context& context,
   auto [cast_type_inst_id, cast_type_id] =
       ExprAsType(context, type_node, parsed_type_id);
 
-  EndSubpatternAsExpr(context, cast_type_inst_id);
+  auto region_id = ConsumeSubpatternExpr(context, cast_type_inst_id);
+  // TODO: Should we be tracking this somewhere?
+  (void)region_id;
 
   auto [name_node, name_id] = context.node_stack().PopNameWithNodeId();