[ELF] - Implemented version script hierarchies.

Patch implements hierarchies for version scripts. 
This allows to handle script files with dependencies, like next one has:

LIBSAMPLE_1.0{
  global:
  a;
};

LIBSAMPLE_2.0
{
  global:
  b;
}LIBSAMPLE_1.0;

Differential revision: http://reviews.llvm.org/D21556

llvm-svn: 273423
diff --git a/lld/ELF/SymbolListFile.cpp b/lld/ELF/SymbolListFile.cpp
index 86e77ed..b9da8b8 100644
--- a/lld/ELF/SymbolListFile.cpp
+++ b/lld/ELF/SymbolListFile.cpp
@@ -94,6 +94,8 @@
     parseVersionSymbols(Version);
 
   expect("}");
+  if (!Version.empty() && peek() != ";")
+    Config->SymbolVersions.back().Parent = next();
   expect(";");
 }