[lldb] Bump the required SWIG version to 3

[lldb] Bump the required SWIG version to 3

Bump the required SWIG version to 3. If my memory serves me well we last
bumped the required SWIG version to 2 for Python 3. At that time SWIG 3
had already been around for a while so everyone I know was already using
that.

It appears that SWIG 3 is the only version that officially supports
C++11 which we're using in the typemap. SWIG 3 was released in 2014 so I
think it's reasonable to make that the minimum required version.

https://bugs.llvm.org/show_bug.cgi?id=48685

Differential revision: https://reviews.llvm.org/D94244