Change struct literal parsing to use placeholders. (#3850)

This is achieving a similar goal as #3849, using placeholders instead of
an ambiguous start node to clarify structure and incrementally simplify
checking. The benefit isn't quite as big here because both paths are
structs, and so checking is more consistent than paren exprs versus
tuples. But I think this removes the only other multi-purpose parse
node.

This uses StructLiteral/StructTypeLiteral naming, reflecting equivalent
SemIR naming. Note, I would lean towards renaming StructLiteral to
StructValueLiteral, but I think consistency in naming takes precedence.
Any renames of StructLiteral might be better in a separate PR.

StructFieldType/StructFieldValue -> StructTypeField/StructField is
trying to making the reading more consistent with
StructTypeLiteral/StructLiteral. SemIR has StructTypeField but not a
value equivalent.

toolchain/parse/node_kind.def

@@ -595,42 +595,40 @@ CARBON_PARSE_NODE_KIND_CHILD_COUNT(IfExprThen, 1, Then)
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(IfExprElse, 3, CARBON_IF_VALID(Else))
 
 // Struct literals, such as `{.a = 0}`:
-//   StructLiteralOrStructTypeLiteralStart
+//   StructLiteralStart
 //         _external_: IdentifierName or BaseName
 //       StructFieldDesignator
 //       _external_: expression
-//     StructFieldValue
+//     StructField
 //     StructComma
 //   _repeated_
 // StructLiteral
 //
 // Struct type literals, such as `{.a: i32}`:
-//   StructLiteralOrStructTypeLiteralStart
+//   StructTypeLiteralStart
 //         _external_: IdentifierName or BaseName
 //       StructFieldDesignator
 //       _external_: type expression
-//     StructFieldType
+//     StructTypeField
 //     StructComma
 //   _repeated_
 // StructTypeLiteral
 //
-// Elements (StructFieldValue and StructFieldType, respectively) and StructComma
+// Elements (StructField and StructTypeField, respectively) and StructComma
 // may repeat with StructComma as a separator.
 //
-// When a valid StructFieldType or StructFieldValue cannot be formed, elements
+// When a valid StructTypeField or StructField cannot be formed, elements
 // may be replaced by InvalidParse, which may have a preceding sibling
 // StructFieldDesignator if one was successfully parsed.
-CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructLiteralOrStructTypeLiteralStart, 0,
-                                   OpenCurlyBrace)
+CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructLiteralStart, 0, OpenCurlyBrace)
+CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructTypeLiteralStart, 0, OpenCurlyBrace)
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructFieldDesignator, 1, Period)
-CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructFieldValue, 2, Equal)
-CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructFieldType, 2, Colon)
+CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructField, 2, Equal)
+CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructTypeField, 2, Colon)
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(StructComma, 0, Comma)
-CARBON_PARSE_NODE_KIND_BRACKET(StructLiteral,
-                               StructLiteralOrStructTypeLiteralStart,
+CARBON_PARSE_NODE_KIND_BRACKET(StructLiteral, StructLiteralStart,
                                CloseCurlyBrace)
-CARBON_PARSE_NODE_KIND_BRACKET(StructTypeLiteral,
-                               StructLiteralOrStructTypeLiteralStart,
+CARBON_PARSE_NODE_KIND_BRACKET(StructTypeLiteral, StructTypeLiteralStart,
                                CloseCurlyBrace)
 
 // Various modifiers. These are all a single token.
@@ -805,15 +803,14 @@ CARBON_PARSE_NODE_KIND_CHILD_COUNT(MatchCaseIntroducer, 0, Case)
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(MatchCaseGuardIntroducer, 0, If)
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(MatchCaseGuardStart, 0,
                                    CARBON_IF_VALID(OpenParen))
-CARBON_PARSE_NODE_KIND_BRACKET(MatchCaseGuard,
-                               MatchCaseGuardIntroducer,
+CARBON_PARSE_NODE_KIND_BRACKET(MatchCaseGuard, MatchCaseGuardIntroducer,
                                CARBON_IF_VALID(CloseParen))
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(MatchCaseEqualGreater, 0,
                                    CARBON_IF_VALID(EqualGreater))
 CARBON_PARSE_NODE_KIND_BRACKET(MatchCaseStart, MatchCaseIntroducer,
-                                   CARBON_IF_VALID(OpenCurlyBrace))
+                               CARBON_IF_VALID(OpenCurlyBrace))
 CARBON_PARSE_NODE_KIND_BRACKET(MatchCase, MatchCaseStart,
-                                   CARBON_IF_VALID(CloseCurlyBrace))
+                               CARBON_IF_VALID(CloseCurlyBrace))
 
 // `default`:
 //     MatchDefaultIntroducer