glcpp: Only warn for macro names containing __

Section 3.3 (Preprocessor) of the GLSL 1.30 spec (and later) and the
GLSL ES spec (all versions) say:

    "All macro names containing two consecutive underscores ( __ ) are
    reserved for future use as predefined macro names. All macro names
    prefixed with "GL_" ("GL" followed by a single underscore) are also
    reserved."

The intention is that names containing __ are reserved for internal use
by the implementation, and names prefixed with GL_ are reserved for use
by Khronos.  Since every extension adds a name prefixed with GL_ (i.e.,
the name of the extension), that should be an error.  Names simply
containing __ are dangerous to use, but should be allowed.  In similar
cases, the C++ preprocessor specification says, "no diagnostic is
required."

Per the Khronos bug mentioned below, a future version of the GLSL
specification will clarify this.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "9.2 10.0 10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Darius Spitznagel <d.spitznagel@goodbytez.de>
Cc: Tapani Pälli <lemody@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71870
Bugzilla: Khronos #11702
(cherry picked from commit 0bd78926304e72ef3566e977d0cb5a959d86b809)
glsl: Only warn for macro names containing __

From page 14 (page 20 of the PDF) of the GLSL 1.10 spec:

    "In addition, all identifiers containing two consecutive underscores
     (__) are reserved as possible future keywords."

The intention is that names containing __ are reserved for internal use
by the implementation, and names prefixed with GL_ are reserved for use
by Khronos.  Names simply containing __ are dangerous to use, but should
be allowed.

Per the Khronos bug mentioned below, a future version of the GLSL
specification will clarify this.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "9.2 10.0 10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Tested-by: Darius Spitznagel <d.spitznagel@goodbytez.de>
Cc: Tapani Pälli <lemody@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71870
Bugzilla: Khronos #11702
(cherry picked from commit 2c85fd5a964a78c9f7a93994fb79f1723c6f45b5)