[Parse] Use StringSwitch to improve readability. NFC

A subsequent patch will propose a "distribute" loop hint.  Similarly to
unroll, this does not have a "assume_safety" argument either so this
condition will get more complex.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266827 91177308-0d34-0410-b5e6-96231b3b80d8