Like clang-check, continue even if some source files were not found in
the compilation database. This makes it possible to do search for
matching sources and query files that exist in the compilation db:
clang-query -c 'm callExpr(callee(functionDecl(hasName("function_name"))))' $(grep -rl ^function_name)
Note: buildASTs calls ClangTool::run which returns 1 if any of the
commands failed and 2 otherwise if any of the files were missing.