Help: Document XLClang compiler id

This compiler id was added by commit 90c6156aa8 (XLClang: Add a new
compiler ID for the clang-based XL compiler, 2019-02-05,
cpp-modules-20190312.1~71^2~7).  Add documentation accidentally left out
of that commit.

Issue: #18835

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -35,6 +35,7 @@ include:
   TI = Texas Instruments (ti.com)
   TinyCC = Tiny C Compiler (tinycc.org)
   XL, VisualAge, zOS = IBM XL (ibm.com)
+  XLClang = IBM Clang-based XL (ibm.com)
 
 This variable is not guaranteed to be defined for all compilers or
 languages.