[ThinLTO] Added a couple of C LTO API interfaces to control the cache policy.

- thinlto_codegen_set_cache_size_bytes to control the absolute size of cache directory. 
- thinlto_codegen_set_cache_size_files the size and amount of files in cache directory. 
These functions have been supported in C++ LTO API for a long time, but were absent in C LTO API.

Differential Revision: https://reviews.llvm.org/D42446

llvm-svn: 326537