Added initial unit test for LLDB's Stream class.

Summary:
This adds an initial small unit test for LLDB's Stream class, which should at least cover
most of the functions in the Stream class. StreamString is always in big endian
mode, so that's the only stream byte order path this test covers as of now. Also,
the binary mode still needs to be tested for all print methods.

Also adds some FIXMEs for wrong/strange result values of the Stream class that we hit
while testing those functions.

Reviewers: labath

Reviewed By: labath

Subscribers: probinson, labath, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D50027

llvm-svn: 338488