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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326537 91177308-0d34-0410-b5e6-96231b3b80d8