[gn build] Slightly simplify write_cmake_config.

Before, the script had a bunch of special cases for #cmakedefine and
#cmakedefine01 and then did general variable substitution. Now, the script
always does general variable substitution for all lines and handles the special
cases afterwards.

This has no observable effect for the inputs we use, but is easier to explain
and slightly easier to implement.

Also mention to link to CMake's configure_file() in the docstring.

(The new behavior doesn't quite match CMake on lines like #cmakedefine ${FOO},
but nobody does that.)

Differential Revision: https://reviews.llvm.org/D55171

llvm-svn: 348106