[LLD][ELF] - Fix warning.

This fixes the following warning when compiling with gcc version 8.0.1 20180319 (experimental) (GCC):

/home/umb/LLVM/llvm/tools/lld/ELF/SyntheticSections.cpp:1951:46: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
     return OS->SectionIndex >= SHN_LORESERVE ? SHN_XINDEX : OS->SectionIndex;

llvm-svn: 340164