[llvm-install-name-tool] Merge install-name options

This diff merges all options for llvm-install-name-tool under a single
function processLoadCommands. Also adds another test case for -add_rpath
option.

Test plan: make check-all

Reviewed by: jhenderson, alexshap, smeenai, Ktwu

Differential Revision: https://reviews.llvm.org/D82812
diff --git a/llvm/tools/llvm-objcopy/CopyConfig.h b/llvm/tools/llvm-objcopy/CopyConfig.h
index ce119de..1341dd6 100644
--- a/llvm/tools/llvm-objcopy/CopyConfig.h
+++ b/llvm/tools/llvm-objcopy/CopyConfig.h
@@ -178,8 +178,8 @@
   std::vector<StringRef> DumpSection;
   std::vector<StringRef> SymbolsToAdd;
   std::vector<StringRef> RPathToAdd;
-  std::vector<std::pair<StringRef, StringRef>> RPathsToUpdate;
-  std::vector<std::pair<StringRef, StringRef>> InstallNamesToUpdate;
+  DenseMap<StringRef, StringRef> RPathsToUpdate;
+  DenseMap<StringRef, StringRef> InstallNamesToUpdate;
   DenseSet<StringRef> RPathsToRemove;
 
   // install-name-tool's id option