Add test for duplicate entrypoints (firebase#15)

Author: bourdakos1

.github/workflows/spm.yml

@@ -20,4 +20,4 @@ jobs:
     - name: tvOS Build Tests
       run: xcodebuild -scheme abseil build -sdk "appletvsimulator" -destination 'platform=tvOS Simulator,name=Apple TV'
     - name: iOS Device Build Tests
-      run: xcodebuild -scheme abseil build -sdk 'iphoneos'      
+      run: xcodebuild -scheme abseil build -sdk 'iphoneos' -destination 'generic/platform=iOS'

SwiftPMTests/build-test/test.cc

@@ -46,3 +46,6 @@
 #include "absl/types/any.h"
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
+
+// Test for duplicate `_main` symbol.
+int main(int argc, char** argv) {}