[overlay] magic-symbols-for-install-name conflicts with @_originallyDefinedIn

The _Builtin_float interfaces were defined in $@rpath/lib/libswiftDarwin.dylib until macOS 10.14.4 and then in usr/lib/swift/libswiftDarwin.dylib after that. We were trying to use magic-symbols-for-install-name for the first one and @_originallyDefinedIn for the second one. However, @_originallyDefinedIn uses the @available introduced version as the first bound for $ld$previous$ which conflicts with the magic-symbols-for-install-name $ld$install_name$. Eventually @_originallyDefinedIn will need to support multiple install names, but until then just use it as-is and stop using magic-symbols-for-install-name.

rdar://122351557

Author: ian-twilightcoder

stdlib/public/ClangOverlays/CMakeLists.txt

@@ -2,8 +2,6 @@ add_swift_target_library(swift_Builtin_float
     ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES}
     IS_SDK_OVERLAY
 
-    "${SWIFT_SOURCE_DIR}/stdlib/linker-support/magic-symbols-for-install-name.c"
-
     GYB_SOURCES
       float.swift.gyb