gengtype: Handle braced initialisers in structs

gengtype: Handle braced initialisers in structs

I have a patch that adds braced initialisers to a GTY structure.
gengtype didn't accept that, because it parsed the "{ ... }" in
" = { ... };" as the end of a statement (as "{ ... }" would be in
a function definition) and so it didn't expect the following ";".

This patch explicitly handles initialiser-like sequences.

Arguably, the parser should also skip redundant ";", but that
feels more like a workaround rather than the real fix.

gcc/
	* gengtype-parse.cc (consume_until_comma_or_eos): Parse "= { ... }"
	as a probable initializer rather than a probable complete statement.