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 <[email protected]>
Cc: "9.2 10.0 10.1" <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Tested-by: Kenneth Graunke <[email protected]>
Reviewed-by: Anuj Phogat <[email protected]>
Tested-by: Darius Spitznagel <[email protected]>
Cc: Tapani Pälli <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71870
Bugzilla: Khronos #11702

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