[llvm-objcopy] Add --keep-symbols option

[kongyi]

Summary

We have all the other variants of -keep-*-symbols, but missing --keep-symbols.

[rupprecht]

Code looks good, but can you add a test for this? Adding something to test/tools/llvm-objcopy/ELF/keep-symbol.test should be fine

[kongyi]

Added tests

llvm/tools/llvm-objcopy/CopyConfig.cpp

Expected<DriverConfig> parseObjcopyOptions(ArrayRef<const char *> ArgsArr) {
   for (auto Arg : InputArgs.filtered(OBJCOPY_strip_unneeded_symbol))
     Config.UnneededSymbolsToRemove.emplace_back(Arg->getValue(), UseRegex);
   for (auto Arg : InputArgs.filtered(OBJCOPY_strip_unneeded_symbols))
     if (Error E = addSymbolsFromFile(Config.UnneededSymbolsToRemove, DC.Alloc,
                                      Arg->getValue(), UseRegex))
       return std::move(E);
   for (auto Arg : InputArgs.filtered(OBJCOPY_keep_symbol))
     Config.SymbolsToKeep.emplace_back(Arg->getValue(), UseRegex);
+  for (auto Arg : InputArgs.filtered(OBJCOPY_keep_symbols))
+    if (Error E = addSymbolsFromFile(Config.SymbolsToKeep, DC.Alloc,
+                                     Arg->getValue(), UseRegex))
+      return std::move(E);
   for (auto Arg : InputArgs.filtered(OBJCOPY_add_symbol)) {
     Expected<NewSymbolInfo> NSI = parseNewSymbolInfo(Arg->getValue());
     if (!NSI)
       return NSI.takeError();
     Config.SymbolsToAdd.push_back(*NSI);
   }
 
   Config.DeterministicArchives = InputArgs.hasFlag(

llvm/tools/llvm-objcopy/ObjcopyOpts.td

: Eq<"strip-symbols",
          "Reads a list of symbols from <filename> and removes them.">,
       MetaVarName<"filename">;
 
 def N : JoinedOrSeparate<["-"], "N">, Alias<strip_symbol>;
 defm keep_symbol : Eq<"keep-symbol", "Do not remove symbol <symbol>">,
                    MetaVarName<"symbol">;
 def K : JoinedOrSeparate<["-"], "K">, Alias<keep_symbol>;
 
+defm keep_symbols
+    : Eq<"keep-symbols",
+         "Reads a list of symbols from <filename> and runs as if "
+         "--keep-symbol=<symbol> is set for each one. <filename> "
+         "contains one symbol per line and may contain comments beginning with "
+         "'#'. Leading and trailing whitespace is stripped from each line. May "
+         "be repeated to read symbols from many files.">,
+      MetaVarName<"filename">;
+
 def only_keep_debug
     : Flag<["-", "--"], "only-keep-debug">,
       HelpText<"Clear sections that would not be stripped by --strip-debug. "
                "Currently only implemented for COFF.">;
 
 def keep_file_symbols : Flag<["-", "--"], "keep-file-symbols">,
                         HelpText<"Do not remove file symbols">;
 defm dump_section