Hold GIL while allocating memory for PythonString.

Summary:
Swig wraps C++ code into SWIG_PYTHON_THREAD_BEGIN_ALLOW; ...  SWIG_PYTHON_THREAD_END_ALLOW;
Thus, LLDB crashes with "Fatal Python error: Python memory allocator called without holding the GIL" when calls an lldb_SB***___str__ function.

Reviewers: clayborg

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D51569

llvm-svn: 341482