Hoist and beef up the asserts about the level of infrastructure expected

when working with a diagnostic attached to a source location. Also
comment more thoroughly why its important to handle non-location
diagnostic messages separately.

Finally, hoist the creation of the TextDiagnostic object up to the
beginning of the location-based diagnostics. This paves the way for
sinking more and more of the logic into this class. When everything
below this constructor is sunk into the TextDiagnostic class it should
be sufficiently "feature complete" to accomplish my two goals:
1) Have the printing of a macro expansion note use the exact same code
   as any other note.
2) Be able to implement clang_formatDiagnostic in terms of this class.

llvm-svn: 140526