[ASTGen] Fix off-by-one error in allocation of UTF-8 string for buffer use
[ASTGen] Fix off-by-one error in allocation of UTF-8 string for buffer use

The semantics of llvm::MemoryBuffer and NULL termination are... subtle.

apple/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,apple/swift,roambotics/swift,roambotics/swift,roambotics/swift,roambotics/swift,roambotics/swift