[SILOpt][OSLog] Skip constant folding StaticString in the
OSLogOptimization pass
The OSLogOptimization pass constant evaluates and folds SIL instructions
that are inlined from the construction of the string interpolations passed
to the log calls, which enables replacing the dynamic format string construction
with a static format string. In addition to folding constant strings, it also folds
constant integers and arrays whose elements are constants. This change makes it
skip folding static strings, since they are already efficiently represented.
rdar://146028438
(cherry picked from commit cbf804e9f34c7fdcdca8c86910a94723df698362)
