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

Start by reading section 3.5.1 of the SDCC user guide and the prior discussion in issue #1821. No source files or tests are named, so first locate the existing handling for compiler-specific declarations, then define completion as IntelliSense recognizing __sfr, __sbit, and related declarations well enough to inspect register definitions.