[ADT] Micro-optimize and force inlining for string switches.

These are now quite heavily used in unit tests and the host tools,
making it worth having them be reasonably fast even in an unoptimized
build. This change reduces the total runtime of TripleTest.Normalization
by yet another 10% to 15%. It is now under 10 seconds on my machine, and
the total check-llvm time has dropped from 38s to around 36s.

I experimented with a number of different options, and the code pattern
here consistently seemed to lower the cleanest, likely due to the
significantly simple CFG and far fewer redundant tests of 'Result'.

llvm-svn: 247257