Add the Z 'type' to the mangling ABI to denote internal symbols.

See discussion here: https://github.com/D-Programming-Language/druntime/pull/725

D already creates mangled names using the Z symbol for internal symbols, e.g.
_D3foo3Bar6__vtblZ for the virtual table for class foo.Bar. The ABI makes no
mention of these symbols. This pull request adds Z to the mangling ABI to make
these official symbols. The linked pull request implements that demangling.
fix Issue 13538 - Divide old C style syntax from D style grammar rule
fix Issue 13539 - Disallow optional template parameters with C-style syntax
Divide DeclaratorIdentifier to VarDeclaratorIdentifier and newly added AltDeclaratorIdentifier
C-style function pointers now an error
Merge pull request #672 from WalterBright/error-fp

C-style function pointers now an error
.min for floating types is an error
use of invariant has been an error for a while
Merge pull request #671 from WalterBright/error-invariant

use of invariant has been an error for a while
Merge pull request #673 from WalterBright/dep-min

.min for floating types is an error
Merge pull request #628 from Poita/bug8578

Add the Z 'type' to the mangling ABI to denote internal symbols.
Merge pull request #664 from 9rnsr/fix_declaration

Issue 13538 & 13539 - Improve variable declaration grammar for old C style syntax