[llvm-mca] Unify the API for the various views. NFCI

This allows the customization of the performance report.

Users can specify their own custom sequence of views.
Each view contributes a portion of the performance report generated by the
BackendPrinter.

Internally, class BackendPrinter keeps a sequence of views; views are printed
out in sequence when method 'printReport()' is called. 

This patch addresses one of the two review comments from Clement in D43951.

llvm-svn: 327018