[clang][Tooling] Fix potential UB in ExpandResponseFilesCompilationDatabase

Summary:
`vector::assign` will cause UB at here.

fixes: https://github.com/clangd/clangd/issues/223

Reviewers: kadircet, sammccall, hokein

Reviewed By: sammccall

Subscribers: merge_guards_bot, ilya-biryukov, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71172