[ELF] - Introduce StripPolicy instead of Config->StripAll/StripDebug flags.

This approach is not only consistent with UnresolvedPolicy, 
but also should help to solve a problem 
of options with opposing meanings, mentioned in PR28843

Differential revision: https://reviews.llvm.org/D23869

llvm-svn: 280206
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index e0fd5f4..71ca4a1 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -323,7 +323,7 @@
     return &InputSection<ELFT>::Discarded;
   }
 
-  if (Config->StripDebug && Name.startswith(".debug"))
+  if (Config->Strip != StripPolicy::None && Name.startswith(".debug"))
     return &InputSection<ELFT>::Discarded;
 
   // The linker merges EH (exception handling) frames and creates a