[libclang] Enhance logging capabilities of libclang.

-provide a "raw_ostream'ish" class to make it convenient to output logging info.
-use macros to automate a bit the logging functionality inside libclang functions
-when logging, print a stack trace if "LIBCLANG_LOGGING=2" environment is set.
-add logging to more functions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172089 91177308-0d34-0410-b5e6-96231b3b80d8