On my system both python-3.4 and python-2.7 are installed.
swift-lldb selects the wrong version (3.4):
...
[6/840] Building CXX object scripts/Python/modules/readline/CMakeFiles/readline.dir/readline.cpp.o
FAILED: /usr/local/bin/clang++ -DHAVE_PROCESS_VM_READV -DHAVE_ROUND -DLIBXML2_DEFINED -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Dreadline_EXPOR
TS -Iscripts/Python/modules/readline -I/var/tmp/lldb/scripts/Python/modules/readline -I/var/tmp/lldb/include -Iinclude -I/var/tmp/build/Ninja-ReleaseAssert/llvm-linux-x86_64/
include -I/var/tmp/build/Ninja-ReleaseAssert/llvm-linux-x86_64/tools/clang/include -I/var/tmp/llvm/include -I/var/tmp/llvm/tools/clang/include -I/var/tmp/build/Ninja-ReleaseA
ssert/swift-linux-x86_64/include -I/var/tmp/swift/include -I/var/tmp/lldb/source -I/usr/include/python3.4 -I/var/tmp/lldb/tools/clang/include -I../clang/include -I/usr/includ
e/libxml2 -Werror=date-time -std=c++11 -fcolor-diagnostics -ffunction-sections -fdata-sections -Wno-deprecated-declarations -Wno-unknown-pragmas -Wno-strict-aliasing -Wno-dep
recated-register -Wno-vla-extension -fno-exceptions -fno-rtti -Wno-macro-redefined -O3 -DNDEBUG -fPIC -MMD -MT scripts/Python/modules/readline/CMakeFiles/readline.dir/readli
ne.cpp.o -MF scripts/Python/modules/readline/CMakeFiles/readline.dir/readline.cpp.o.d -o scripts/Python/modules/readline/CMakeFiles/readline.dir/readline.cpp.o -c /var/tmp/ll
db/scripts/Python/modules/readline/readline.cpp
/var/tmp/lldb/scripts/Python/modules/readline/readline.cpp:68:34: error: assigning to 'char *(*)(FILE *, FILE *, const char *)' (aka 'char *(*)(_IO_FILE *, _IO_FILE *, const
char *)') from incompatible type 'char *(FILE *, FILE *, char *)' (aka 'char *(_IO_FILE *, _IO_FILE *, char *)'): type mismatch at 3rd parameter ('const char *' vs 'char *')
PyOS_ReadlineFunctionPointer = simple_readline;
^ ~~~~~~~~~~~~~~~
/var/tmp/lldb/scripts/Python/modules/readline/readline.cpp:70:5: error: use of undeclared identifier 'Py_InitModule4'
Py_InitModule4(
^
2 errors generated.