[llvm-objcopy] Rename --keep to --keep-section.

Summary:
llvm-objcopy/strip support `--keep` (for sections) and `--keep-symbols` (for symbols). For consistency and clarity, rename `--keep` to `--keep-section`.
In fact, for GNU compatability, -K is --keep-symbol, so it's weird that the alias `-K` is not the same as the short-ish `--keep`.

Reviewers: jakehehrlich, jhenderson, alexshap, MaskRay, espindola

Reviewed By: jakehehrlich, MaskRay

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D54477

llvm-svn: 346782
diff --git a/llvm/tools/llvm-objcopy/CopyConfig.h b/llvm/tools/llvm-objcopy/CopyConfig.h
index 7ebe2a0..68cd247 100644
--- a/llvm/tools/llvm-objcopy/CopyConfig.h
+++ b/llvm/tools/llvm-objcopy/CopyConfig.h
@@ -57,7 +57,7 @@
   // Repeated options
   std::vector<StringRef> AddSection;
   std::vector<StringRef> DumpSection;
-  std::vector<StringRef> Keep;
+  std::vector<StringRef> KeepSection;
   std::vector<StringRef> OnlyKeep;
   std::vector<StringRef> SymbolsToGlobalize;
   std::vector<StringRef> SymbolsToKeep;