[ELF] - Fix build bot.

SyntheticSections.cpp:1773:29: error: chosen constructor is explicit in copy-initialization
        CuVectors.push_back({});
                            ^~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/set:428:14: note: constructor declared here
    explicit set(const value_compare& __comp = value_compare())
             ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/vector:702:59: note: passing argument to parameter '__x' here
    _LIBCPP_INLINE_VISIBILITY void push_back(value_type&& __x);

llvm-svn: 305054