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)
fix Issue 12420 - [AA] Can't set associative array with array as key value using key type

And add test case for issue 2954.