Extract an isReservedName() function

Extract an isReservedName() function

We'll want to share the implementation if anything else decides to check
for reserved names in future, so make this little snippet of code more
discoverable.

Also remove the __va_list_tag and __builtin_va_list special-case
checks. They're leftovers from before when the reserved name logic was
added.

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212006 91177308-0d34-0410-b5e6-96231b3b80d8