d: Use read() to load contents of stdin into memory.

d: Use read() to load contents of stdin into memory.

This would be an improvement over reading one character at a time.

An ICE was discovered when mixing reading from stdin with `-v', this has been
fixed in upstream DMD and backported as well.

Reviewed-on: https://github.com/dlang/dmd/pull/11620

gcc/d/ChangeLog:

	* d-lang.cc (d_parse_file): Use read() to load contents from stdin,
	allow the front-end to free the memory after parsing.
	* dmd/MERGE: Merge upstream dmd 2cc25c219.