[analyzer] [NFC] Remove unused Extensive diagnostic setting,

Rename AlternateExtensive to Extensive.
In 2013, five years ago, we have switched to AlternateExtensive
diagnostics by default, and Extensive was available under unused,
undocumented flag.
This change remove the flag, renames the Alternate
diagnostic to Extensive (as it's no longer Alternate), and ports the
test.

Differential Revision: https://reviews.llvm.org/D47670

llvm-svn: 334524