[LTO] Fix a use-after-free in legacy LTO C APIs (llvm#107896)

Fix a bug that `lto_runtime_lib_symbols_list` is returning the address
of a local variable that will be freed when getting out of scope. This
is a regression from llvm#98512 that rewrites the runtime libcall function
lists into a SmallVector.

rdar://135559037

Author: cachemeifyoucan

llvm/tools/lto/lto.cpp

@@ -13,6 +13,7 @@
 
 #include "llvm-c/lto.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/CodeGen/CommandFlags.h"
@@ -88,6 +89,8 @@ struct LTOToolDiagnosticHandler : public DiagnosticHandler {
   }
 };
 
+static SmallVector<const char *> RuntimeLibcallSymbols;
+
 // Initialize the configured targets if they have not been initialized.
 static void lto_initialize() {
   if (!initialized) {
@@ -108,6 +111,7 @@ static void lto_initialize() {
     LTOContext = &Context;
     LTOContext->setDiagnosticHandler(
         std::make_unique<LTOToolDiagnosticHandler>(), true);
+    RuntimeLibcallSymbols = lto::LTO::getRuntimeLibcallSymbols(Triple());
     initialized = true;
   }
 }
@@ -691,7 +695,6 @@ extern const char *lto_input_get_dependent_library(lto_input_t input,
 }
 
 extern const char *const *lto_runtime_lib_symbols_list(size_t *size) {
-  auto symbols = lto::LTO::getRuntimeLibcallSymbols(Triple());
-  *size = symbols.size();
-  return symbols.data();
+  *size = RuntimeLibcallSymbols.size();
+  return RuntimeLibcallSymbols.data();
 }