libsyntax: Disallow keywords followed by `::`.

This breaks code that looked like:

    mymacro!(static::foo);

... where `mymacro!` expects a path or expression. Change such macros to
not accept keywords followed by `::`.

Closes #17298.

[breaking-change]
auto merge of #17379 : pcwalton/rust/keywords-followed-by-double-colon, r=brson

This breaks code that looked like:

    mymacro!(static::foo);

... where `mymacro!` expects a path or expression. Change such macros to
not accept keywords followed by `::`.

Closes #17298.

[breaking-change]