[lldb][cmake] Remove custom logic for finding VCS file to fix LLDB's VCSVersion.inc generation

[teemperor]

Summary

We are still implementing our own logic for this that looks for a VCS file in the place where it
was before the monorepo migration. This removes this logic and just uses the CMake function
that LLVM/Clang are using.

[teemperor]

See clang/lib/Basic/CMakeLists.txt for the respective Clang code.

lldb/source/CMakeLists.txt

 include_directories(.)
 
 set(lldbBase_SOURCES
     lldb.cpp
   )
 
-foreach(file
-        "${LLDB_SOURCE_DIR}/.git/logs/HEAD" # Git
-        "${LLDB_SOURCE_DIR}/.svn/wc.db"     # SVN 1.7
-        "${LLDB_SOURCE_DIR}/.svn/entries"   # SVN 1.6
-        )
-  if(EXISTS "${file}")
-    set(lldb_vc "${file}")
-    break()
-  endif()
-endforeach()
+
+find_first_existing_vc_file("${LLDB_SOURCE_DIR}" lldb_vc)
 
 set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/VCSVersion.inc")
 set(generate_vcs_version_script "${LLVM_CMAKE_PATH}/GenerateVersionFromVCS.cmake")
 
 if(lldb_vc AND LLVM_APPEND_VC_REV)
   set(lldb_source_dir ${LLDB_SOURCE_DIR})
 endif()