Fix compiler Test Registry to use a StringMap.

We have no guarantee that the StringRef passed to the registry lives
in the static data segment.

This resulted in memory corruption during bootstrapping, which is
particularly difficult to diagnose and reproduce.

Author: atrick

lib/SIL/Utils/Test.cpp

@@ -25,7 +25,7 @@ using namespace swift::test;
 namespace {
 
 class Registry {
-  DenseMap<StringRef, FunctionTest *> registeredTests;
+  StringMap<FunctionTest *> registeredTests;
   SwiftNativeFunctionTestThunk thunk;
 
 public:
@@ -47,7 +47,8 @@ class Registry {
   SwiftNativeFunctionTestThunk getFunctionTestThunk() { return thunk; }
 
   FunctionTest *getFunctionTest(StringRef name) {
-    auto *res = registeredTests[name];
+    // Avoid creating a new entry here.
+    auto *res = registeredTests.lookup(name);
     if (!res) {
       llvm::errs() << "Found no test named " << name << "!\n";
       print(llvm::errs());
@@ -58,8 +59,9 @@ class Registry {
   void print(raw_ostream &OS) const {
     OS << "test::Registry(" << this << ") with " << registeredTests.size()
        << " entries: {{\n";
-    for (auto pair : registeredTests) {
-      OS << "\t" << pair.getFirst() << " -> " << pair.getSecond() << "\n";
+    for (auto &stringMapEntry : registeredTests) {
+      OS << "\t" << stringMapEntry.getKey() << " -> "
+         << stringMapEntry.getValue() << "\n";
     }
     OS << "}} test::Registry(" << this << ")\n";
   }