StringOptimization: optimize interpolated C strings.
Optimize code like:
puts("\(String.self)")
Optimizing string interpolation and optimizing C-strings are both done in StringOptimization.
A second run of the StringOptimization is needed in the pipeline to optimize such code, because the result of the interpolation-optimization must be cleaned up so that the C-String optimization can kick in.
Also, StringOptimization must handle struct_extract(struct(literal)), where the struct_extract may be in a called function.
To solve a phase ordering problem with inlining String semantics and inlining the `String(stringInterpolation: DefaultStringInterpolation)` constructor, we do a simple analysis of the callee. Doing this simple "interprocedural" analysis avoids relying on inlining that String constructor.
rdar://74941849
