Make the function to compute the prebuilt module cache path public

so it can be called by LLDB. (NFC).

rdar://76112632
(cherry picked from commit ab3da16)

Author: adrian-prantl

include/swift/Frontend/Frontend.h

@@ -204,6 +204,12 @@ class CompilerInvocation {
 
   void setRuntimeResourcePath(StringRef Path);
 
+  /// Compute the default prebuilt module cache path for a given resource path
+  /// and SDK version. This function is also used by LLDB.
+  static std::string
+  computePrebuiltCachePath(StringRef RuntimeResourcePath, llvm::Triple target,
+                           Optional<llvm::VersionTuple> sdkVer);
+
   /// If we haven't explicitly passed -prebuilt-module-cache-path, set it to
   /// the default value of <resource-dir>/<platform>/prebuilt-modules.
   /// @note This should be called once, after search path options and frontend

lib/Frontend/CompilerInvocation.cpp

@@ -101,28 +101,34 @@ getVersionedPrebuiltModulePath(Optional<llvm::VersionTuple> sdkVer,
   } while(true);
 }
 
-void CompilerInvocation::setDefaultPrebuiltCacheIfNecessary() {
-
-  if (!FrontendOpts.PrebuiltModuleCachePath.empty())
-    return;
-  if (SearchPathOpts.RuntimeResourcePath.empty())
-    return;
-
-  SmallString<64> defaultPrebuiltPath{SearchPathOpts.RuntimeResourcePath};
+std::string CompilerInvocation::computePrebuiltCachePath(
+    StringRef RuntimeResourcePath, llvm::Triple target,
+    Optional<llvm::VersionTuple> sdkVer) {
+  SmallString<64> defaultPrebuiltPath{RuntimeResourcePath};
   StringRef platform;
-  if (tripleIsMacCatalystEnvironment(LangOpts.Target)) {
-    // The prebuilt cache for macCatalyst is the same as the one for macOS, not iOS
-    // or a separate location of its own.
+  if (tripleIsMacCatalystEnvironment(target)) {
+    // The prebuilt cache for macCatalyst is the same as the one for macOS, not
+    // iOS or a separate location of its own.
     platform = "macosx";
   } else {
-    platform = getPlatformNameForTriple(LangOpts.Target);
+    platform = getPlatformNameForTriple(target);
   }
   llvm::sys::path::append(defaultPrebuiltPath, platform, "prebuilt-modules");
 
   // If the SDK version is given, we should check if SDK-versioned prebuilt
   // module cache is available and use it if so.
-  FrontendOpts.PrebuiltModuleCachePath =
-    getVersionedPrebuiltModulePath(LangOpts.SDKVersion, defaultPrebuiltPath);
+  return getVersionedPrebuiltModulePath(sdkVer, defaultPrebuiltPath);
+}
+
+void CompilerInvocation::setDefaultPrebuiltCacheIfNecessary() {
+
+  if (!FrontendOpts.PrebuiltModuleCachePath.empty())
+    return;
+  if (SearchPathOpts.RuntimeResourcePath.empty())
+    return;
+
+  FrontendOpts.PrebuiltModuleCachePath = computePrebuiltCachePath(
+      SearchPathOpts.RuntimeResourcePath, LangOpts.Target, LangOpts.SDKVersion);
 }
 
 static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,