request to support sdcc mcs51 declarations (__sfr, __sbit, etc)

sdcc, which is quite popular open-source, free embedded compiler, uses special extensions for intel 8051 (aka mcs51) define special function registers and other similar things (like code, ram space, etc)
e.g.

__sfr __at (0xC0) P4   ;
__sbit __at (0xC0) P4_0 ;

extensions are discussed further in section 3.5.1 of sdcc user guide:
http://sdcc.sourceforge.net/doc/sdccman.pdf

We are using sdcc for a new platformio intel-mcs51 platform, (https://github.com/platformio/platform-intel_mcs51) and would love to be able to use the vscode cpptools intellisense here to peek at register definitions, etc.

I see this was previously discussed and rejected for keil C51 compiler at #1821, however I think this is quite different in that Keil is closed/proprietary/non-free (and has it's own ide) whereas sdcc is completely free and opensource, and sdcc+platformio+vscode+cpptools are getting really close to making full open-source ide stack

tagging a few stakeholders:
@ivankravets @spth @znhocn

Beginne mit dem Lesen von Abschnitt 3.5.1 des SDCC user guide und der vorherigen Diskussion in issue #1821. Da keine Quelldateien oder Tests genannt sind, ermittle zunächst die bestehende Behandlung compiler-spezifischer Deklarationen und definiere anschließend die Fertigstellung so, dass IntelliSense __sfr, __sbit und entsprechende Deklarationen gut genug erkennt, um Registerdefinitionen zu untersuchen.