[PECOFF] Fix /export option in the .drectve section.

/EXPORT option has slightly different semantics if it appears in the .drectve
section. This patch implements it.

llvm-svn: 197970
diff --git a/lld/lib/ReaderWriter/PECOFF/PECOFFLinkingContext.cpp b/lld/lib/ReaderWriter/PECOFF/PECOFFLinkingContext.cpp
index ba5bbfc..8363f25 100644
--- a/lld/lib/ReaderWriter/PECOFF/PECOFFLinkingContext.cpp
+++ b/lld/lib/ReaderWriter/PECOFF/PECOFFLinkingContext.cpp
@@ -219,6 +219,13 @@
   return allocate(str);
 }
 
+StringRef PECOFFLinkingContext::undecorateSymbol(StringRef name) const {
+  if (_machineType != llvm::COFF::IMAGE_FILE_MACHINE_I386)
+    return name;
+  assert(name.startswith("_"));
+  return name.substr(1);
+}
+
 Writer &PECOFFLinkingContext::writer() const { return *_writer; }