Pull the parsing helper functions out of the Triple interface entirely.

Pull the parsing helper functions out of the Triple interface entirely.
They're private static methods but we can just make them static
functions in the implementation. It makes the implementations a touch
more wordy, but takes another chunk out of the header file.

Also, take the opportunity to switch the names to the new coding
conventions.

No functionality changed here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151047 91177308-0d34-0410-b5e6-96231b3b80d8