[clang][driver] Fix compilation database dump with multiple architectures

Command lines with multiple `-arch` arguments expand into multiple entries in the compilation database. However, the file writes are not appending, meaning subsequent writes end up overwriting the previous ones, resulting in garbled output.

This patch fixes that by always appending to the file.

rdar://90165004

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D121997