Workaround FD leaks from clang module files in performSema

Until we get a proper fix for rdar://problem/19720334 to close the .pcm
files immediately after reading them, forcefully close all the .pcm file
descriptors at the end of performSema.  This avoids file descriptor
leaks that eventually bring down SourceKit.

rdar://problem/19059478

Swift SVN r25038

lib/Frontend/Frontend.cpp

@@ -26,6 +26,8 @@
 #include "swift/Parse/Lexer.h"
 #include "swift/SIL/SILModule.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
+#include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/Preprocessor.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Support/CommandLine.h"
@@ -221,6 +223,22 @@ Module *CompilerInstance::getMainModule() {
   return MainModule;
 }
 
+namespace {
+class CloseClangModuleFiles {
+  clang::Preprocessor &PP;
+public:
+  CloseClangModuleFiles(clang::Preprocessor &PP) : PP(PP) {}
+  ~CloseClangModuleFiles() {
+    clang::ModuleMap &ModMap = PP.getHeaderSearchInfo().getModuleMap();
+    for (auto I = ModMap.module_begin(), E = ModMap.module_end(); I != E; ++I) {
+      clang::Module *M = I->second;
+      if (!M->isSubModule() && M->getASTFile())
+        M->getASTFile()->closeFile();
+    }
+  }
+};
+} // end anonymous namespace
+
 void CompilerInstance::performSema() {
   const FrontendOptions &options = Invocation.getFrontendOptions();
   const SourceFileKind Kind = Invocation.getInputKind();
@@ -254,6 +272,11 @@ void CompilerInstance::performSema() {
   auto clangImporter =
     static_cast<ClangImporter *>(Context->getClangModuleLoader());
 
+  // When we exit, we won't build any more clang modules, so close the .pcm
+  // files to prevent fd leaks in clients that cache the AST.
+  // FIXME: Remove this once rdar://problem/19720334 is complete.
+  CloseClangModuleFiles ccmf(clangImporter->getClangPreprocessor());
+
   Module *underlying = nullptr;
   if (options.ImportUnderlyingModule) {
     underlying = clangImporter->loadModule(SourceLoc(),