Fix positional output argument for llvm-strip.

[srhines]

Summary

This was accidentally clobbering the input file, and never creating the
output file when using a second positional input parameter.

[srhines]

Switch to using .back() because I was lazy in the first upload. :)

[alexander-shaposhnikov]

@srhines - i've just checked binutils strip again, so basically the positional arguments here are not "input" + "output", instead,
it's a list of "input" files which should be modified in-place.
For example, strip a.o b.o c.o should strip each of them.

[srhines]

Gah, you are correct. Our Android build system had grown to depend on this behavior from llvm-strip previously. Doing an update now is how I am finding all of these issues. I'll fix it in our build system and abandon this CL. Thanks for the review.

test/tools/llvm-objcopy/strip-all.test

 # RUN: cp %t %t7
 # RUN: llvm-strip -strip-all %t7
 # RUN: cmp %t2 %t7
 
 # RUN: cp %t %t8
 # RUN: llvm-strip --strip-all %t8
 # RUN: cmp %t2 %t8
 
+# Verify that a positional output file leaves the input unchanged.
+# RUN: cp %t %t10
+# RUN: llvm-strip --strip-all %t10 %t11
+# RUN: cmp %t %t10
+# RUN: cmp %t2 %t11
+
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64
 Sections:
   - Name:            .bss

tools/llvm-objcopy/llvm-objcopy.cpp

if (Positional.empty())
     error("No input file specified");
 
   if (Positional.size() > 2)
     error("Support for multiple input files is not implemented yet");
 
   CopyConfig Config;
   Config.InputFilename = Positional[0];
   Config.OutputFilename =
-      InputArgs.getLastArgValue(STRIP_output, Positional[0]);
+      InputArgs.getLastArgValue(STRIP_output, Positional.back());
 
   Config.StripDebug = InputArgs.hasArg(STRIP_strip_debug);
 
   Config.DiscardAll = InputArgs.hasArg(STRIP_discard_all);
   Config.StripUnneeded = InputArgs.hasArg(STRIP_strip_unneeded);
   Config.StripAll = InputArgs.hasArg(STRIP_strip_all);
 
   if (!Config.StripDebug && !Config.StripUnneeded && !Config.DiscardAll)