[lldb] Unifying lldb python path

Based on mgorny@'s D67890

There are 3 places where python site-package path is calculated
independently:

1. finishSwigPythonLLDB.py where files are written to site-packages.

2. lldb/scripts/CMakeLists.txt where site-packages are installed.

3. ScriptInterpreterPython.cpp where site-packages are added to
PYTHONPATH.

This change creates the path once and use it everywhere. So that they
will not go out of sync.

Also it provides a chance for cross compiling users to specify the right
path for site-packages.

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68442

llvm-svn: 373991

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.h

@@ -48,8 +48,7 @@ public:
 
 protected:
   static void ComputePythonDirForApple(llvm::SmallVectorImpl<char> &path);
-  static void ComputePythonDirForPosix(llvm::SmallVectorImpl<char> &path);
-  static void ComputePythonDirForWindows(llvm::SmallVectorImpl<char> &path);
+  static void ComputePythonDir(llvm::SmallVectorImpl<char> &path);
 };
 } // namespace lldb_private