Add functions to save and restore the PrettyStackTrace state.

PrettyStackTraceHead is a LLVM_THREAD_LOCAL, which means it's just a global
in LLVM_ENABLE_THREADS=NO builds.  If a CrashRecoveryContext is used with
code that uses PrettyStackEntries, and a crash happens, PrettyStackTraceHead is
currently not reset to its pre-crash value.  These functions make it possible
to add a cleanup to such code that does this.

(Not reseting the value then causes the assert in ~PrettyStackTraceEntry() to
fire if the code outside of the CrashRecoveryContext also uses
PrettyStackEntries -- for example, clang when building a module.)

Part of PR11974.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244338 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/PrettyStackTrace.cpp

@@ -154,6 +154,20 @@ void llvm::EnablePrettyStackTrace() {
 #endif
 }
 
+const void* llvm::SavePrettyStackState() {
+#if defined(HAVE_BACKTRACE) && defined(ENABLE_BACKTRACES)
+  return PrettyStackTraceHead;
+#else
+  return nullptr;
+#endif
+}
+
+void llvm::RestorePrettyStackState(const void* Top) {
+#if defined(HAVE_BACKTRACE) && defined(ENABLE_BACKTRACES)
+  PrettyStackTraceHead = (const PrettyStackTraceEntry*)Top;
+#endif
+}
+
 void LLVMEnablePrettyStackTrace() {
   EnablePrettyStackTrace();
 }