[PrefixMap] Remap clang cc1 args when building swift interfaces

Fix a bug that swift clang importer is not setup correctly when prefix
map is used. There are two separate issues:
* CC1 args used to setup clang import when building swift (interface and
  sources) are not correctly remapped.
* When loading SDKInfo from SDK path, the SDKSettings.json is not
  loading from VFS, thus the file cannot be loaded from remapped path.

rdar://134458611

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1787,6 +1787,9 @@ void InterfaceSubContextDelegateImpl::inheritOptionsForBuildingInterface(
   genericSubInvocation.getClangImporterOptions().ClangImporterDirectCC1Scan =
       clangImporterOpts.ClangImporterDirectCC1Scan;
 
+  genericSubInvocation.getSearchPathOptions().ScannerPrefixMapper =
+      SearchPathOpts.ScannerPrefixMapper;
+
   // Validate Clang modules once per-build session flags must be consistent
   // across all module sub-invocations
   if (clangImporterOpts.ValidateModulesOnce) {