[clang][docs] Remove trailing whitespace

Which is causing CI checks to fail.

clang/docs/LanguageExtensions.rst:2794:takes no arguments and produces an unsigned long long result. The builtin does
clang/docs/LanguageExtensions.rst:2795:not guarantee any particular frequency, only that it is stable. Knowledge of the
+ echo '*** Trailing whitespace has been found in Clang source files as described above ***'