[next] Convert const char * fields to StringRef

llvm/llvm-project d0262c2394f46bb7da2a75529413d625c70908e5 added a new
default bool param to the two constructors in `SmallVectorMemoryBuffer`.
Since `options.OutputPath` is a `const char *` and that can be promoted
to a `bool`, the constructor being called was changed to the first
constructor (with a default buffer name) - promotion is preferred over
conversion.

Convert the various output paths to a `StringRef` - all their uses
converted to `StringRef` anyway. Also specify the default parameter in
order to maintain the old behaviour, which didn't require a null
terminator.

lib/Frontend/ModuleInterfaceBuilder.cpp

@@ -273,7 +273,7 @@ bool ModuleInterfaceBuilder::buildSwiftModuleInternal(
     // optimization pipeline.
     SerializationOptions SerializationOpts;
     std::string OutPathStr = OutPath.str();
-    SerializationOpts.OutputPath = OutPathStr.c_str();
+    SerializationOpts.OutputPath = OutPathStr;
     SerializationOpts.ModuleLinkName = FEOpts.ModuleLinkName;
     SerializationOpts.AutolinkForceLoad =
       !subInvocation.getIRGenOptions().ForceLoadSymbolName.empty();