[doc] Use cmake's -S option to simplify the build instructions

[ebraminio]

Summary

Now that llvm's minimum version is 3.13.4 anyway and -S is
introduced in the same version[1], let's use this little
time saving option to simplify the build instructions.

[1]: https://cgold.readthedocs.io/en/latest/glossary/-S.html

[ebraminio]

Thanks for the review :) So guess can be merged now?

[evandro]

In D95049#2530687, @ebraminio wrote:

Thanks for the review :) So guess can be merged now?

Yes, once accepted, one can commit a patch under review.

[ebraminio]

From https://llvm.org/docs/Phabricator.html

It is sufficient to add a comment to the approved review indicating you cannot commit the patch yourself.

"I can't commit the patch myself" So here it is :)

P.S.

Wished to upload some other patches in the meanwhile but got busy with other things yet feel this minor thing reduces time to begin for others.

README.md

 
      * ``cd llvm-project``
 
-     * ``mkdir build``
-
-     * ``cd build``
-
-     * ``cmake -G <generator> [options] ../llvm``
+     * ``cmake -S llvm -B build -G <generator> [options]``
 
         Some common build system generators are:
 

         * ``-DLLVM_ENABLE_ASSERTIONS=On`` --- Compile with assertion checks enabled
           (default is Yes for Debug builds, No for all other build types).
 
-      * ``cmake --build . [-- [options] <target>]`` or your build system specified above
+      * ``cmake --build build [-- [options] <target>]`` or your build system specified above
         directly.
 
         * The default target (i.e. ``ninja`` or ``make``) will build all of LLVM.