[4.8] Backport LTO fixes and improvements

Squashed commit of r198172, 198173, 198174, 198736,
and 198741 from trunk.

Notable fixes and improvements:
  * -flto-report-wpa has been introduced to only
    print for WPA.
  * Print the LTO report earlier so type merging
    hash tables are shown.
  * Fix LTO report names.
  * Private names are no longer supported/added.