Add a compiler SPI for "description without comments"

Introduce a new `SyntaxProtocol.descriptionWithoutComments` that removes
comments from the given syntax, replacing them with either a space (if
the comment has no newlines) or a newline (if it does have newlines).

Author: DougGregor

Sources/SwiftIfConfig/ActiveSyntaxRewriter.swift

@@ -379,3 +379,38 @@ extension IfConfigDeclSyntax {
     }
   }
 }
+
+extension SyntaxProtocol {
+  // Produce the source code for this syntax node with all of the comments
+  // removed. Each comment will be replaced with either a newline or a space,
+  // depending on whether the comment involved a newline.
+  @_spi(Compiler)
+  public var descriptionWithoutComments: String {
+    var result = ""
+    for token in tokens(viewMode: .sourceAccurate) {
+      token.leadingTrivia.writeWithoutComments(to: &result)
+      token.text.write(to: &result)
+      token.trailingTrivia.writeWithoutComments(to: &result)
+    }
+    return result
+  }
+}
+
+extension Trivia {
+  fileprivate func writeWithoutComments(to stream: inout some TextOutputStream) {
+    for piece in pieces {
+      switch piece {
+      case .backslashes, .carriageReturnLineFeeds, .carriageReturns, .formfeeds, .newlines, .pounds, .spaces, .tabs,
+        .unexpectedText, .verticalTabs:
+        piece.write(to: &stream)
+
+      case .blockComment(let text), .docBlockComment(let text), .docLineComment(let text), .lineComment(let text):
+        if text.contains(where: \.isNewline) {
+          stream.write("\n")
+        } else {
+          stream.write(" ")
+        }
+      }
+    }
+  }
+}

Tests/SwiftIfConfigTest/VisitorTests.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import SwiftDiagnostics
-import SwiftIfConfig
+@_spi(Compiler) import SwiftIfConfig
 import SwiftParser
 import SwiftSyntax
 @_spi(XCTestFailureLocation) @_spi(Testing) import SwiftSyntaxMacrosGenericTestSupport
@@ -299,6 +299,42 @@ public class VisitorTests: XCTestCase {
         """
     )
   }
+
+  func testRemoveComments() {
+    let original: SourceFileSyntax = """
+
+      /// This is a documentation comment
+      func f() { }
+
+      /** Another documentation comment
+          that is split across
+          multiple lines */
+      func g() { }
+
+      func h() {
+        x +/*comment*/y
+        // foo
+      }
+      """
+
+    assertStringsEqualWithDiff(
+      original.descriptionWithoutComments,
+      """
+
+       
+      func f() { }
+
+
+
+      func g() { }
+
+      func h() {
+        x + y
+         
+      }
+      """
+    )
+  }
 }
 
 /// Assert that applying the given build configuration to the source code