Maybe this will help. Had the same exact issue, and it was this setting having an explicit value set:
The equivalent setting in your project file will be:
GCC_VERSION = com.apple.compilers.llvm.clang.1_0;
Removing this explicit setting (letting it use the default, which should be the same exact value) caused all of the Swift Compiler settings to show back up. You can either remove this line from your xcodeproj manually, or select it as in the screenshot above, and hit delete