[DTLTO] [LLVM] Initial DTLTO cache implementation (#156433)

This patch implements DTLTO cache. DTLTO cache is implemented the same
way as ThinLTO cache. In fact the same class Cache is used for both of
them.

 Because parameters for codegen are different for DTLTO and ThinLTO
 (DTLTO codegen is done by invoking clang and its codegen parameters are
 not fully synchronized with codegen parameters used by LTO backend).
 The object files generated by DTLTO and ThinLTO might be different and
 shouldn't be mixed. If ThinLTO and DTLTO share the same cache
 directory, the cache file won't interfere with each other.

 I added a couple of test files in cross-project-test/dtlto directory,
 but if more tests are required for initial implementation, I could add
 them.

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