Introduce a new variant type 'TryResult' to represent the result of

Introduce a new variant type 'TryResult' to represent the result of
TryEvaluateBool instead of using a raw 'int'.  This avoids any
confusion of how 'int' converts to bool, and makes the resultant code
easier to read.

Condense a bunch of 'addSuccessor()' calls in 'if ... else' to use the
ternary operator instead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76947 91177308-0d34-0410-b5e6-96231b3b80d8