[clang-cl] Implement /external:I, /external:env, and EXTERNAL_INCLUDE support (PR36003)

This patch does three things:

- Map the /external:I flag to -isystem

- Add support for the /external:env:<var> flag which reads system
  include paths from the <var> environment variable

- Pick up system include dirs EXTERNAL_INCLUDE in addition to the old
  INCLUDE environment variable.

Differential revision: https://reviews.llvm.org/D104387