[NFC] [clang-tidy] Fix potential SA issues. (#170289)
This patch addresses issues identified by the static analyzers, which
appear to be legitimate problems.
`FloatLoopCounterCheck.cpp`: "Dereferencing a pointer that might be
`nullptr` FS when calling `getInc`".
`ProBoundsAvoidUncheckedContainerAccessCheck.cpp`: "Dereferencing a
pointer that might be `nullptr Callee` when calling `getBeginLoc`".
`ExpandModularHeadersPPCallbacks.cpp`: Non-static class member
`CurrentToken.Flags` is not initialized in this constructor nor in any
functions that it calls. (line #101).
