[llvm] [cmake] Use current directory in GenerateVersionFromVCS

[mgorny]

Summary

Find dependent scripts of GenerateVersionFromVCS in current directory
rather than ../../cmake/modules. I do not see any reason why the former
would not work and The latter is incorrect when GenerateVersionFromVCS
is used from install directory (i.e. in stand-alone builds).

[phosek]

LGTM

llvm/cmake/modules/GenerateVersionFromVCS.cmake

 # CMake script that writes version control information to a header.
 #
 # Input variables:
 #   NAMES             - A list of names for each of the source directories.
 #   <NAME>_SOURCE_DIR - A path to source directory for each name in NAMES.
 #   HEADER_FILE       - The header file to write
 #
 # The output header will contain macros <NAME>_REPOSITORY and <NAME>_REVISION,
 # where "<NAME>" is substituted with the names specified in the input variables,
 # for each of the <NAME>_SOURCE_DIR given.
 
-get_filename_component(LLVM_DIR "${CMAKE_SCRIPT_MODE_FILE}" PATH)
-get_filename_component(LLVM_DIR "${LLVM_DIR}" PATH)
-get_filename_component(LLVM_DIR "${LLVM_DIR}" PATH)
+get_filename_component(LLVM_CMAKE_DIR "${CMAKE_SCRIPT_MODE_FILE}" PATH)
 
-list(APPEND CMAKE_MODULE_PATH "${LLVM_DIR}/cmake/modules")
+list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
 
 include(VersionFromVCS)
 
 # Handle strange terminals
 set(ENV{TERM} "dumb")
 
 function(append_info name path)
   if(path)