[Dependency Scanning] Specify Explicit Module output path to the scanner explicitly

Instead of the code querying the compiler's built-in Clang instance, refactor the
dependency scanner to explicitly keep track of module output path. It is still
set according to '-module-cache-path' as it has been prior to this change, but
now the scanner can use a different module cache for scanning PCMs, as specified
with '-clang-scanner-module-cache-path', without affecting module output path.

Resolves rdar://113222853

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -174,7 +174,7 @@ void ClangImporter::recordModuleDependencies(
 
     auto pcmPath = moduleCacheRelativeLookupModuleOutput(
         clangModuleDep.ID, ModuleOutputKind::ModuleFile,
-        getModuleCachePathFromClang(getClangInstance()));
+        cache.getModuleOutputPath());
     swiftArgs.push_back("-o");
     swiftArgs.push_back(pcmPath);
 
@@ -421,7 +421,7 @@ ClangImporter::getModuleDependencies(StringRef moduleName,
   }
   std::string workingDir = *optionalWorkingDir;
 
-  auto moduleCachePath = getModuleCachePathFromClang(getClangInstance());
+  auto moduleCachePath = cache.getModuleOutputPath();
   auto lookupModuleOutput =
       [moduleCachePath](const ModuleID &MID,
                         ModuleOutputKind MOK) -> std::string {