Switch the llvm::Triple class to immediately parse the triple string on

Switch the llvm::Triple class to immediately parse the triple string on
construction. Simplify its interface, implementation, and users
accordingly as there is no longer an 'uninitialized' state to check for.
Also, fixes a bug lurking in the interface as there was one method that
didn't correctly check for initialization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151024 91177308-0d34-0410-b5e6-96231b3b80d8