Xcode 6.4 Release was released on 30 Jun 2015 and requires macOS 10.10+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_6.4/Xcode_6.4.dmg.
Release notes are located at https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW364.
The SDKs it contains are:
- iOS 12H141
- macOS 13F34
- macOS 14D125
The compilers it contains are:
- clang 6.1.0 (Build 602.0.53)
- swift 1.2 (Build 602.0.53.1)
- LLVM-GCC 3.6.0 (Build )