Crash in lld with thin LTO cache and assembly output

[quic-akaryaki]

After #136121 is merged, thin LTO with assembly output with cache crashes lld. A test case is basic:

int main() { return 0; }

clang -c main.c -O2 -flto=thin -o main.o
ld.lld --thinlto-cache-dir=. --lto-emit-asm -e main main.o

The cache directory should not have cached output files.

The output stream in LTOBackend: codegen() is of type CacheStream, defined in Caching.cpp. The commit function of this class deletes the underlying raw_pwrite_stream (OS). With LTO and assembly output, MCAsmStreamer is created as an output streamer and it also holds a raw pointer to raw_pwrite_stream indirectly through a formatted_raw_output wrapper. In the MCAsmStreamer destructor, formatted_raw_ostream calls flush() on a deleted object.

A simple fix would be to delete the LTO pipeline before the stream commit function is called in LTOBackend.cpp: codegen().