Introduce separate header for SwiftCompilerSupport library.

This is more staging to move the compiler over to SwiftCompilerSupport.

Author: DougGregor

Package.swift

@@ -127,7 +127,11 @@ let package = Package(
     .target(
       name: "SwiftCompilerSupport",
       dependencies: [
-        "SwiftSyntax", "SwiftParser", "SwiftDiagnostics", "SwiftOperators"]
+        "SwiftSyntax", "SwiftParser", "SwiftDiagnostics", "SwiftOperators"],
+      exclude: [
+        "CMakeLists.txt",
+        "SwiftCompilerSupport.h"
+      ]
     ),
     .executableTarget(
       name: "lit-test-helper",

Sources/SwiftCompilerSupport/SwiftCompilerSupport.h

@@ -0,0 +1,52 @@
+/*===----------------------- SwiftCompilerSupport.h -------------------------===
+
+   This source file is part of the Swift.org open source project
+
+   Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+   Licensed under Apache License v2.0 with Runtime Library Exception
+
+   See https://swift.org/LICENSE.txt for license information
+   See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+  ===----------------------------------------------------------------------===*/
+#ifndef SWIFT_COMPILER_SUPPORT_H
+#define SWIFT_COMPILER_SUPPORT_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// Flags used to describe consistency checks that can be performed by
+/// the
+enum SwiftConsistencyCheckFlags {
+  /// Ensure that the syntax tree produced by the parser fully reproduces the
+  /// input source.
+  SCC_RoundTrip = 0x01,
+
+  /// Check for the presence of parsing diagnostics.
+  SCC_ParseDiagnostics = 0x02,
+
+  /// Perform sequence folding on the syntax tree.
+  SCC_FoldSequences = 0x04,
+};
+
+/// Entry point for the Swift compiler to use for consistency checking.
+///
+/// - Parameters:
+///   - bufferPtr: Pointer to the input buffer.
+///   - bufferLength: Length of the input buffer.
+///   - filename: The name of the source file, which is used only for diagnostics
+///   - flags: Flags that indicate what checks should be performed.
+///       0x01: Perform round-trip checking.
+/// - Returns: 0 if all requested consistency checks passed, nonzero otherwise.
+int swift_parser_consistencyCheck(
+  const char *buffer, ptrdiff_t bufferLength, const char *filename,
+  unsigned int flags);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SWIFT_COMPILER_SUPPORT_H */