Bump version to 15.0.50.DATE-git.

Bump version to 15.0.50.DATE-git.

Now that the GDB 14 branch has been created,
this commit bumps the version number in gdb/version.in to
15.0.50.DATE-git

For the record, the GDB 14 branch was created
from commit 8f12a1a841cd0c447de7a5a0f134a0efece73588.

Also, as a result of the version bump, the following changes
have been made in gdb/testsuite:

	* gdb.base/default.exp: Change $_gdb_major to 15.