Make CMake display more readable paths to Python binaries on Windows

Summary:
Previously CMake would display messages like these:

```
-- LLDB Found PythonExecutable: $<$<CONFIG:Debug>:C:/Projects/Python-2.7.9-bin/x64/python_d.exe>$<$<NOT:$<CONFIG:Debug>>:C:/Projects/Python-2.7.9-bin/x64/python.exe>
-- LLDB Found PythonLibs: $<$<CONFIG:Debug>:C:/Projects/Python-2.7.9-bin/x64/libs/python27_d.lib>$<$<NOT:$<CONFIG:Debug>>:C:/Projects/Python-2.7.9-bin/x64/libs/python27.lib>
-- LLDB Found PythonDLL: $<$<CONFIG:Debug>:C:/Projects/Python-2.7.9-bin/x64/python27_d.dll>$<$<NOT:$<CONFIG:Debug>>:C:/Projects/Python-2.7.9-bin/x64/python27.dll>
```

This patch makes the messages look like this:

```
-- LLDB Found PythonExecutable: C:/Projects/Python-2.7.9-bin/x64/python.exe and C:/Projects/Python-2.7.9-bin/x64/python_d.exe
-- LLDB Found PythonLibs: C:/Projects/Python-2.7.9-bin/x64/libs/python27.lib and C:/Projects/Python-2.7.9-bin/x64/libs/python27_d.lib
-- LLDB Found PythonDLL: C:/Projects/Python-2.7.9-bin/x64/python27.dll and C:/Projects/Python-2.7.9-bin/x64/python27_d.dll
```

I've also added checks to ensure the messages are actually accurate, as in check that the files actually exist before claiming they've been found. If any of the files are missing Python integration will be disabled for the build.

Patch by Vadim Macagon. Thanks!

Reviewers: brucem, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13520

llvm-svn: 249671