Add -vgc switch

The following known allocations are intentionally not handled:

* AssertExp
* CmpExp
* AssignExp: Calls _d_arrayassign
* Hidden function errors: Are already deprecated
* SwitchErrorStatement

All these cases only allocate by throwing exceptions
and disallowing them would impact user code a lot. In
order to really solve such issues we need some way to
throw exceptions without GC-allocation.

Also not handled are indirect calls to user-defined
functions which may allocate (calling postblits in
array assignment for example). As this commit does
not provide a way to mark a function as @nogc there's
no way to know if such functions actually allocate.

These indirect calls to user defined functions must
be revisited when a @nogc attribute is introduced.
Don't indent location part of diagnostic messages
Merge pull request #3402 from CyberShadow/ambig-indent

Don't indent location part of diagnostic messages
Don't rebuild d_do_test.exe every time on Windows
Merge pull request #3403 from CyberShadow/d_do_test_exe

Don't rebuild d_do_test.exe every time on Windows
Merge pull request #1886 from jpf91/vgc

Implement nogc framework (warn about GC allocations)
Add "ambiguous type" and ready to use it
fix Issue 1983 - Delegates violate const

Carry information which function is overloaded and use it properly.
fix Issue 7322 - Taking address of deprecated functions isn't refused