Remove dependency from raw_ostream on <chrono>

The tryLockFor method from raw_fd_sotreamis the sole user of that
header, and it's not referenced in the mono repo. I still chose to keep
it (may be useful for downstream user) but added a transient type that's
forward declared to hold the duration parameter.

Notable changes:

- "llvm/Support/Duration.h" must be included in order to use tryLockFor.
- "llvm/Support/raw_ostream.h" no longer includes <chrono>

This sole change has an interesting impact on the number of processed
line, as measured by:

clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/Support/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l

before: 7917500
after:  7835142

Discourse thread on the topic: https://llvm.discourse.group/t/include-what-you-use-include-cleanup/5831