[clangd][test] Fix build error of FeatureModulesTests

clang-tools-extra/clangd/unittests/FeatureModulesTests.cpp:33:58: error:
could not convert ‘(const char*)""’ from ‘const char*’ to
llvm::StringLiteral’
       llvm::StringLiteral kind() const override { return ""; };

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D100612