Use access levels on imports when compiler(>=6).

Access levels on `import` declarations are a language feature that was
introduced with the Swift 6 compiler but they don't depend on the Swift 6
language mode. `@_implementationOnly import` is now deprecated and `internal
import` should be used instead, so SwiftSyntax's use of access levels on
imports should depend on the compiler version, not the language version to
ensure it no longer uses the `@_implementationOnly` unnecessarily.

As a notable exception, `_SwiftSyntaxCShims` must continue to be imported
`@_implementationOnly` since it's a local module that cannot be loaded by
clients, and `internal import` does not hide modules from dependents when
SwiftSyntax is built without library evolution.

Author: tshortli

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparser/IsLexerClassifiedFile.swift

@@ -18,7 +18,7 @@ import Utils
 let isLexerClassifiedFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     public import SwiftSyntax
     #else
     import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparser/LayoutNodesParsableFile.swift

@@ -18,7 +18,7 @@ import Utils
 let layoutNodesParsableFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(RawSyntax) public import SwiftSyntax
     #else
     @_spi(RawSyntax) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparser/ParserTokenSpecSetFile.swift

@@ -29,7 +29,7 @@ func tokenCaseMatch(
 let parserTokenSpecSetFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(RawSyntax) @_spi(ExperimentalLanguageFeatures) public import SwiftSyntax
     #else
     @_spi(RawSyntax) @_spi(ExperimentalLanguageFeatures) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparser/TokenSpecStaticMembersFile.swift

@@ -18,7 +18,7 @@ import Utils
 let tokenSpecStaticMembersFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(RawSyntax) internal import SwiftSyntax
     #else
     @_spi(RawSyntax) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparserdiagnostics/ChildNameForDiagnosticsFile.swift

@@ -18,7 +18,7 @@ import Utils
 let childNameForDiagnosticFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(ExperimentalLanguageFeatures) internal import SwiftSyntax
     #else
     @_spi(ExperimentalLanguageFeatures) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparserdiagnostics/SyntaxKindNameForDiagnosticsFile.swift

@@ -18,7 +18,7 @@ import Utils
 let syntaxKindNameForDiagnosticFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(ExperimentalLanguageFeatures) internal import SwiftSyntax
     #else
     @_spi(ExperimentalLanguageFeatures) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparserdiagnostics/TokenNameForDiagnosticsFile.swift

@@ -18,7 +18,7 @@ import Utils
 let tokenNameForDiagnosticFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(RawSyntax) internal import SwiftSyntax
     #else
     @_spi(RawSyntax) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntaxbuilder/BuildableNodesFile.swift

@@ -18,7 +18,7 @@ import Utils
 let buildableNodesFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(ExperimentalLanguageFeatures) public import SwiftSyntax
     #else
     @_spi(ExperimentalLanguageFeatures) import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntaxbuilder/RenamedChildrenBuilderCompatibilityFile.swift

@@ -18,7 +18,7 @@ import Utils
 let renamedChildrenBuilderCompatibilityFile = try! SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     public import SwiftSyntax
     #else
     import SwiftSyntax

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntaxbuilder/ResultBuildersFile.swift

@@ -18,7 +18,7 @@ import Utils
 let resultBuildersFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   DeclSyntax(
     """
-    #if swift(>=6)
+    #if compiler(>=6)
     @_spi(ExperimentalLanguageFeatures) public import SwiftSyntax
     #else
     @_spi(ExperimentalLanguageFeatures) import SwiftSyntax