Model function calls as initializing expressions (#3089)

Start treating function calls as initializing expressions instead of as
value expressions.

This required adding support for expression categories. Value bindings
and temporary materialization conversions are created where necessary to
transition between expression categories. For a function call with a
return slot, we speculatively create a materialized temporary before the
call and either commit to it or replace it with something else later,
once we see how the function call expression is actually used.

This change follows the direction suggested in #3133 for initializing
expressions: depending on the return type of a function, the return
value will either be initialized in-place or returned directly. This is
visible in the semantics IR, which is a little unfortunate but is
probably necessary as this is part of the semantics of the program.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/semantics/semantics_handle_struct.cpp

@@ -48,6 +48,11 @@ auto HandleStructFieldValue(Context& context, ParseTree::Node parse_node)
       context.node_stack().PopExpressionWithParseNode();
   SemIR::StringId name_id = context.node_stack().Pop<ParseNodeKind::Name>();
 
+  // Convert the operand to a value.
+  // TODO: We need to decide how struct literals interact with expression
+  // categories.
+  value_node_id = context.ConvertToValueExpression(value_node_id);
+
   // Store the name for the type.
   auto type_block_id = context.args_type_info_stack().PeekForAdd();
   context.semantics_ir().AddNode(