[Clang] Use const pointer to eliminate warning with libxml 2.12.0 (#76719)

Currently, if `CLANG_HAVE_LIBXML` is defined, and the version of libxml2
is above 2.12.0, there will be two warnings when building clang.

warning: initializing 'xmlErrorPtr' (aka 'struct _xmlError *') with an
expression of type 'const xmlError *' (aka 'const struct _xmlError *')
discards qualifiers

Since this commit

https://gitlab.gnome.org/GNOME/libxml2/-/commit/45470611b047db78106dcb2fdbd4164163c15ab7,
libxml2 makes cmlGetLastError return a const error. This patch follows
libxml2. Making the result a const pointer should be compatible with
versions before 2.12.0.

Tested on ArchLinux with libxml2 2.12.3 installed.