libphobos: Fallback on UnwindBacktrace if LibBacktrace not defined.

In the gcc.backtrace module, either one of LibBacktrace or
UnwindBacktrace will always be defined.  Giving UnwindBacktrace a higher
precedence over the libc backtrace as the default handler because the
latter depends on a rt.backtrace module that is not compiled in.

libphobos/ChangeLog:

	* libdruntime/core/runtime.d (defaultTraceHandler): Give
	UnwindBacktrace handler precedence over backtrace.

From-SVN: r268836

libphobos/ChangeLog

+2019-02-13  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* libdruntime/core/runtime.d (defaultTraceHandler): Give
+	UnwindBacktrace handler precedence over backtrace.
+
 2019-02-10  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	* libdruntime/Makefile.am (DRUNTIME_DSOURCES): Remove rt/util/hash.d

libphobos/libdruntime/core/runtime.d

@@ -619,6 +619,22 @@ Throwable.TraceInfo defaultTraceHandler( void* ptr = null )
         }
         return new LibBacktrace(FIRSTFRAME);
     }
+    else static if ( __traits( compiles, new UnwindBacktrace(0) ) )
+    {
+        version (Posix)
+        {
+            static enum FIRSTFRAME = 5;
+        }
+        else version (Win64)
+        {
+            static enum FIRSTFRAME = 4;
+        }
+        else
+        {
+            static enum FIRSTFRAME = 0;
+        }
+        return new UnwindBacktrace(FIRSTFRAME);
+    }
     else static if ( __traits( compiles, backtrace ) )
     {
         import core.demangle;
@@ -885,22 +901,6 @@ Throwable.TraceInfo defaultTraceHandler( void* ptr = null )
         auto s = new StackTrace(FIRSTFRAME, cast(CONTEXT*)ptr);
         return s;
     }
-    else static if ( __traits( compiles, new UnwindBacktrace(0) ) )
-    {
-        version (Posix)
-        {
-            static enum FIRSTFRAME = 5;
-        }
-        else version (Win64)
-        {
-            static enum FIRSTFRAME = 4;
-        }
-        else
-        {
-            static enum FIRSTFRAME = 0;
-        }
-        return new UnwindBacktrace(FIRSTFRAME);
-    }
     else
     {
         return null;