[ADT] Force inline several super boring and unusually hot methods on

SmallVector to further help debug builds not waste their time calling
one line functions.

To give you an idea of why this is worthwhile, this change alone gets
another >10% reduction in the runtime of TripleTest.Normalization! It's
now under 9 seconds for me. Sadly, this is the end of the easy wins for
that test. Anything further will require some different architecture of
the test itself. Still, I'm pretty happy. 'check-llvm' now is under 35s
for me.

llvm-svn: 247259