SR-11411: Update GettingStarted.md

[spevans]

Update the document to reflect current cmake usage.

@compnerd @millenomi This is based on the commands I use, do you have anything to add/alter?

[compnerd]

Thanks for adding the blurb on running individual tests, thats extremely valuable. LGTM (might be nice to wait a day or so to see if @millenomi chimes in)

[spevans]

@swift-ci test and merge

[compnerd]

macOS builds are known to be broken due to the dysmutil issue. Im going to merge this as is. If you know how to convert the build from dwarf-with-dsym to dwarf, would you mind putting up a PR to switch modes?

[spevans]

The macOS build is actually pointless in this repo as according to build-scrip-impl:

if [[ ${host} == "macosx"* ]]; then
    echo "Skipping Foundation on OS X -- use the Xcode project instead"
    continue
fi