clang-scan-deps should be aware of the output file

[mathstuf]

clang-scan-deps currently writes to stdout. CMake issue #25429 was filed because the naïve > output solution unconditionally updates the output file even if scanning fails. If it was a known output, a failed scan could reliably delete the output file so that make and ninja understand the state properly.

Cc: @ChuanqiXu9 @iains @Bigcheese