Structure R600's computeDataLayout more like every other target.

While there, simplify "p3:32:32:32" to "p3:32:32".
Maybe add new warning for shadowing simple tag types
Make Sema::BuildCXXNestedNameSpecifier correctly clear the previous
CXXScopeSpec when necessary while performing typo correction. This fixes
the crash reported in PR18213 (the problem existed since r185487, and
r193020 made it easier to hit).