[ELF] Support PROVIDE/PROVIDE_HIDDEN inside output sections description.

Differential Revision:  https://reviews.llvm.org/D22738

llvm-svn: 276579
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index cfc6f29..2ddfd9c 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -676,6 +676,10 @@
         InCmd->Patterns.push_back(next());
       }
       expect(")");
+    } else if (Tok == "PROVIDE") {
+      readProvide(false);
+    } else if (Tok == "PROVIDE_HIDDEN") {
+      readProvide(true);
     } else {
       setError("unknown command " + Tok);
     }