glcpp: Fix source numbers set with "#line LINE_NUMBER SOURCE_NUMBER"

glcpp: Fix source numbers set with "#line LINE_NUMBER SOURCE_NUMBER"

Previously, the YY_USER_ACTION was overwriting the yylloc->source value
in every action, (after that value had been carefully set by the handling
of the #line directive). Instead, we want to initialize it once in
YY_USER_INIT and then not touch it at all in YY_USER_ACTION.