Since commit b6367f723b (Swift/Ninja: Emit modules separately from
compilation, 2026-04-10) the compile edge still appends
-emit-module -emit-module-path when emitModuleSeparately is active,
duplicating the emit-module edge. For targets with directory-style
module paths (CMP0195=NEW), this causes a file/directory conflict
that fails the build.
Guard -emit-module -emit-module-path on both the compile and
emit-module edges so they are not appended when the flags already
contain -emit-module-path (e.g. from the target's compile options).
Discovered building swift-syntax on Windows where the directory-style
path was overwritten by the flat path from the compile edge.
Issue: #27748
