glcpp: Avoid unnecessary call to strlen

glcpp: Avoid unnecessary call to strlen

Length of the token was already calculated by flex and stored in yyleng,
no need to implicitly call strlen() via linear_strdup().

Signed-off-by: Thomas Helland <thomashelland90@gmail.com>
Tested-by: Dieter Nützel <Dieter at nuetzel-hh.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>
V2: Also convert this pattern in glsl_lexer.ll

V3: Remove a misplaced comment

V4: Use a temporary char to avoid type change
    Remove bogus +1 on length check of identifier