Fix narrowing conversion error in 32-bit MSVC builds

Use size_t instead of ELFT::uint for the string table offset. If the
linker is built 32-bit, it can't write an output file larger than 2GB.
Other code in this area uses size_t as well.

llvm-svn: 284680
diff --git a/lld/ELF/OutputSections.h b/lld/ELF/OutputSections.h
index f73c418..f5a5270 100644
--- a/lld/ELF/OutputSections.h
+++ b/lld/ELF/OutputSections.h
@@ -266,7 +266,7 @@
 
 struct SymbolTableEntry {
   SymbolBody *Symbol;
-  std::size_t StrTabOffset;
+  size_t StrTabOffset;
 };
 
 template <class ELFT>