pre

Allow 'ninja -t compdb' accept targets

Test:
./ninja -t compdb -a wrong_target (fail)
./ninja -t compdb -a test_target (empty due to lack of rules)
./ninja -t compdb -a test_target cc cxx (ok)
./ninja -t compdb cxx (ok, regression test)
./ninja ninja_test && ./ninja_test (passed)

Co-authored-by: Linkun Chen <[email protected]>
Co-authored-by: csmoe <[email protected]>