Unsupported variable expansion for user variables

Type: Debugger

Describe the bug

• OS and Version:
  Linux - Fedora 31 Linux 5.5.11-200.fc31.x86_64 #1 SMP Mon Mar 23 17:32:43 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

• VS Code Version:
  Version: 1.43.2
  Commit: 0ba0ca52957102ca3527cf479571617f0de6ed50
  Date: 2020-03-24T07:52:11.516Z
  Electron: 7.1.11
  Chrome: 78.0.3904.130
  Node.js: 12.8.1
  V8: 7.8.279.23-electron.0
  OS: Linux x64 5.5.11-200.fc31.x86_64

• C/C++ Extension Version:
  C/C++ for Visual Studio Code Change Log
  Version 0.27.0-insiders5: March 26, 2019

• Other extensions you installed (and if the issue persists after disabling them):
  N/A

• A clear and concise description of what the bug is.

If I set an environment variable that depends upon another variable, it is not dereferenced correctly

Example:

"environment": [
{"name":"VAR1",
"value": "${workspaceFolder}"},
{"name":"VAR2",
"value": "${VAR1}"},]

The result is: ],
VAR1=/home/jack/dev/
VAR2=${VAR1}

Expected result is:
VAR1=/home/jack/dev/
VAR2=/home/jack/dev/

Built-in variables such ${workspaceFolder} are dereferenced (and any other number of built-ins too) there's no reason the same parser can't look for all other extant environment variables too.

Beware infinite loop -
Should also support:
"environment": [
{"name":"VAR", "value": "SOME_VALUE"},
{"name":"VAR", "value": "${VAR} OTHER_VALUE"},
]
To produce VAR="SOME_VALUE OTHER_VALUE"

Since the variable (e.g. ${workspaceFolder}) syntax is already defined as "${VARIABLE}" this should be trivial.

Start by tracing how the debugger processes the configuration's environment entries and expands ${...} variables. Reproduce the VAR1/VAR2 example and the self-referential VAR example from the issue. Done means user-defined variables expand to their resolved values, including references to earlier values, without an infinite loop.