[libFuzzer] Don't prefix absolute paths in fuchsia.

The ExecuteCommand function in fuchsia used to prefix the
getOutputFile for each command run with the artifact_prefix flag if
it was available, because fuchsia components don't have a writable working
directory. However, if a file with a global path is provided, fuchsia
should honor that.

An example of this is using the global /tmp directory to store stuff.
In fuchsia it ended up being translated to data///tmp, whereas we want
to make sure it is using /tmp (which is available to components using the
isolated-temp feature).

To test this I made the change, compiled fuchsia with this toolchain and
ran a fuzzer with the -fork=1 flag (that mode makes use of the /tmp
directory). I also tested that normal fuzzing workflow was not affected
by this.

Author: charco (Marco Vanotti)
Differential Revision: https://reviews.llvm.org/D68774

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@374612 91177308-0d34-0410-b5e6-96231b3b80d8

Author: jakehehrlich

FuzzerUtilFuchsia.cpp

@@ -407,13 +407,14 @@ int ExecuteCommand(const Command &Cmd) {
   // that lacks a mutable working directory. Fortunately, when this is the case
   // a mutable output directory must be specified using "-artifact_prefix=...",
   // so write the log file(s) there.
+  // However, we don't want to apply this logic for absolute paths.
   int FdOut = STDOUT_FILENO;
   if (Cmd.hasOutputFile()) {
-    std::string Path;
-    if (Cmd.hasFlag("artifact_prefix"))
-      Path = Cmd.getFlagValue("artifact_prefix") + "/" + Cmd.getOutputFile();
-    else
-      Path = Cmd.getOutputFile();
+    std::string Path = Cmd.getOutputFile();
+    bool IsAbsolutePath = Path.length() > 1 && Path[0] == '/';
+    if (!IsAbsolutePath && Cmd.hasFlag("artifact_prefix"))
+      Path = Cmd.getFlagValue("artifact_prefix") + "/" + Path;
+
     FdOut = open(Path.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
     if (FdOut == -1) {
       Printf("libFuzzer: failed to open %s: %s\n", Path.c_str(),