Remove TimeValue class

Summary:
All usages have been replaced by appropriate std::chrono funcionality, and the
class is now unused. The only used part of the cpp file is the DumpTimePoint
function, which I have moved into the only caller (CommandObjectTarget.cpp).

Reviewers: clayborg, zturner

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D26451

llvm-svn: 287096