Unify Darwin and Non-Darwin printing of version output

Summary:
This change unifies and simplifies the code paths between the Darwin and non-Darwin code to print the LLDB version information.

It also introduces a new variable in CMake LLDB_VERSION_STRING which can be used to specify custom version information. On Darwin this value is implicitly set based on the resource/LLDB-Info.plist file.

With the LLDB_VERSION_STRING variable set to lldb-360.99.0, the -version output is:

> ./bin/lldb -version
lldb version 4.0.0 (lldb-360.99.0)
  clang revision 286264
  llvm revision 286265

This behavior is unified across all target platforms.

Reviewers: lldb-commits

Subscribers: mgorny, tfiala

Differential Revision: https://reviews.llvm.org/D26478

llvm-svn: 286479