Add a test for correctness of C++ compilation.

This required the following:

- Adding Platform_base_sdk_extension_version to default soong.variables
- Teaching the symlink tree creation code to understand symlinks
- Making finder.go follow symlinks when requested

Adding yet another knob is unfortunate, but I can't allow that
unconditionally because the Android code base contains a number of
symlinks giving rise to infinite directory trees because they point back
to their parent and this seemed preferable to adding complicated logic
like "follow symlink but if only its fully resolved version does not
point under the source tree".

I could be convinced about the latter, though.

Test: Presubmits.
Change-Id: I453f6b7e5334771f5832c700db00f9d24ed1d82f

ui/build/finder.go

@@ -64,6 +64,7 @@ func NewSourceFinder(ctx Context, config Config) (f *finder.Finder) {
 	cacheParams := finder.CacheParams{
 		WorkingDirectory: dir,
 		RootDirs:         []string{"."},
+		FollowSymlinks:   config.environ.IsEnvTrue("ALLOW_BP_UNDER_SYMLINKS"),
 		ExcludeDirs:      []string{".git", ".repo"},
 		PruneFiles:       pruneFiles,
 		IncludeFiles: []string{