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/function.cpp

@@ -126,8 +126,9 @@ static auto MakeFunctionSignature(Context& context, SemIR::LocId loc_id,
     context.full_pattern_stack().StartImplicitParamList();
 
     BeginSubpattern(context);
-    auto self_type_region_id = EndSubpatternAsExpr(
+    auto self_type_region_id = ConsumeSubpatternExpr(
         context, context.types().GetTypeInstId(args.self_type_id));
+    EndEmptySubpattern(context);
 
     insts.self_param_id = AddParamPattern(
         context, loc_id, SemIR::NameId::SelfValue, self_type_region_id,
@@ -147,8 +148,9 @@ static auto MakeFunctionSignature(Context& context, SemIR::LocId loc_id,
     context.inst_block_stack().Push();
     for (auto param_type_id : args.param_type_ids) {
       BeginSubpattern(context);
-      auto param_type_region_id = EndSubpatternAsExpr(
+      auto param_type_region_id = ConsumeSubpatternExpr(
           context, context.types().GetTypeInstId(param_type_id));
+      EndEmptySubpattern(context);
 
       context.inst_block_stack().AddInstId(AddParamPattern(
           context, loc_id, SemIR::NameId::Underscore, param_type_region_id,