[Clang importer] Fix the use-after-free with enum info harder.

This is more of rdar://problem/24869070; StringRefs are leaking out of
importFullName still, in a DenseMap that can get deallocated. When we
cache this data, put them in "permanent" (Swift ASTContext) storage.

Author: DougGregor

lib/ClangImporter/ImportEnumInfo.cpp

@@ -167,7 +167,8 @@ static StringRef getCommonPluralPrefix(StringRef singular, StringRef plural) {
 
 /// Determine the prefix to be stripped from the names of the enum constants
 /// within the given enum.
-void EnumInfo::determineConstantNamePrefix(const clang::EnumDecl *decl) {
+void EnumInfo::determineConstantNamePrefix(ASTContext &ctx,
+                                           const clang::EnumDecl *decl) {
   switch (getKind()) {
   case EnumKind::Enum:
   case EnumKind::Options:
@@ -281,5 +282,5 @@ void EnumInfo::determineConstantNamePrefix(const clang::EnumDecl *decl) {
     commonPrefix = commonPrefix.slice(0, commonWithEnum.size() + delta);
   }
 
-  constantNamePrefix = commonPrefix;
+  constantNamePrefix = ctx.AllocateCopy(commonPrefix);
 }

lib/ClangImporter/ImportEnumInfo.h

@@ -17,6 +17,7 @@
 #ifndef SWIFT_CLANG_IMPORT_ENUM_H
 #define SWIFT_CLANG_IMPORT_ENUM_H
 
+#include "swift/AST/ASTContext.h"
 #include "swift/AST/Decl.h"
 #include "clang/AST/Attr.h"
 #include "clang/Basic/SourceLocation.h"
@@ -60,7 +61,7 @@ class EnumInfo {
 
   /// The enum's common constant name prefix, which will be stripped from
   /// constants
-  std::string constantNamePrefix = StringRef();
+  StringRef constantNamePrefix = StringRef();
 
   /// The identifying attribute for specially imported enums
   ///
@@ -71,9 +72,10 @@ class EnumInfo {
 public:
   EnumInfo() = default;
 
-  EnumInfo(const clang::EnumDecl *decl, clang::Preprocessor &pp) {
+  EnumInfo(ASTContext &ctx, const clang::EnumDecl *decl,
+           clang::Preprocessor &pp) {
     classifyEnum(decl, pp);
-    determineConstantNamePrefix(decl);
+    determineConstantNamePrefix(ctx, decl);
   }
 
   EnumKind getKind() const { return kind; }
@@ -92,7 +94,7 @@ class EnumInfo {
   }
 
 private:
-  void determineConstantNamePrefix(const clang::EnumDecl *);
+  void determineConstantNamePrefix(ASTContext &ctx, const clang::EnumDecl *);
   void classifyEnum(const clang::EnumDecl *, clang::Preprocessor &);
 };
 }

lib/ClangImporter/ImporterImpl.h

@@ -497,15 +497,15 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     // If there is no name for linkage, the computation is trivial and we
     // wouldn't be able to perform name-based caching anyway.
     if (!decl->hasNameForLinkage())
-      return importer::EnumInfo(decl, preprocessor);
+      return importer::EnumInfo(SwiftContext, decl, preprocessor);
 
     SmallString<32> keyScratch;
     auto key = getEnumInfoKey(decl, keyScratch);
     auto known = enumInfos.find(key);
     if (known != enumInfos.end())
       return known->second;
 
-    importer::EnumInfo enumInfo(decl, preprocessor);
+    importer::EnumInfo enumInfo(SwiftContext, decl, preprocessor);
     enumInfos[key] = enumInfo;
     return enumInfo;
   }