glsl: Consistently use length-based ralloc string functions for info_log.

glsl: Consistently use length-based ralloc string functions for info_log.

Commit b823b99ec0f13af257dcd885f436a4d294c6222a switched from using
functions such as ralloc_asprintf and ralloc_strcat to
ralloc_asprintf_rewrite_tail. This change maintains the string's
length as a aparamter that is updated by the ralloc functions (rather
than recomputing it with strlen over and over).

However, the change failed to updated two locations (glcpp_error and
glcpp_warning), with the result that the string's length wasn't
updated by these calls. Then, subsequent calls to other
ralloc_asprintf_rewrite_tail would overwrite the text appended by
glcpp_error.

This commit fixes the two missing updates, and restores line numbers
to the output of glcpp error messages, (as noticed by a glcpp unit
test case that has been failing since the above-mentioned commit).

Signed-off-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>