[CodeGen][Timers] Enable llvm::TimePassesIsEnabled when -ftime-report is specified

r330571 added a new FrontendTimesIsEnabled variable and replaced many usages of llvm::TimePassesIsEnabled. Including the place that set llvm::TimePassesIsEnabled for -ftime-report. The effect of this is that -ftime-report now only contains the timers specifically referenced in CodeGenAction.cpp and none of the timers in the backend.

This commit adds back the assignment, but otherwise leaves everything else unchanged.

llvm-svn: 339281