Give NamedRegionTimer an Enabled flag, allowing all its clients to

Give NamedRegionTimer an Enabled flag, allowing all its clients to
switch from this:

  if (TimePassesIsEnabled) {
    NamedRegionTimer T(Name, GroupName);
    do_something();
  } else {
    do_something(); // duplicate the code, this time without a timer!
  }

to this:

  {
    NamedRegionTimer T(Name, GroupName, TimePassesIsEnabled);
    do_something();
  }

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106285 91177308-0d34-0410-b5e6-96231b3b80d8