[clang][clangd] Don't crash/assert on -gsplit-dwarf=single without output

The crash happens in clang::driver::tools::SplitDebugName when Output is
InputInfo::Nothing. It doesn't happen with standalone clang driver because
output is created in Driver::BuildJobsForActionNoCache.

Example backtrace:
```
* thread #1, name = 'clangd', stop reason = hit program assert
  * frame #0: 0x00007ffff5c4eacf libc.so.6`raise + 271
    frame #1: 0x00007ffff5c21ea5 libc.so.6`abort + 295
    frame #2: 0x00007ffff5c21d79 libc.so.6`__assert_fail_base.cold.0 + 15
    frame #3: 0x00007ffff5c47426 libc.so.6`__assert_fail + 70
    frame #4: 0x000055555dc0923c clangd`clang::driver::InputInfo::getFilename(this=0x00007fffffff9398) const at InputInfo.h:84:5
    frame #5: 0x000055555dcd0d8d clangd`clang::driver::tools::SplitDebugName(JA=0x000055555f6c6a50, Args=0x000055555f6d0b80, Input=0x00007fffffff9678, Output=0x00007fffffff9398) at CommonArgs.cpp:1275:40
    frame #6: 0x000055555dc955a5 clangd`clang::driver::tools::Clang::Constru...