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

finder/finder.go

@@ -94,6 +94,10 @@ type CacheParams struct {
 	// RootDirs are the root directories used to initiate the search
 	RootDirs []string
 
+	// Whether symlinks are followed. If set, symlinks back to their own parent
+	// directory don't work.
+	FollowSymlinks bool
+
 	// ExcludeDirs are directory names that if encountered are removed from the search
 	ExcludeDirs []string
 
@@ -1415,9 +1419,14 @@ func (f *Finder) listDirSync(dir *pathMap) {
 				// If stat fails this is probably a broken or dangling symlink, treat it as a file.
 				subfiles = append(subfiles, child.Name())
 			} else if childStat.IsDir() {
-				// Skip symlink dirs.
-				// We don't have to support symlink dirs because
-				// that would cause duplicates.
+				// Skip symlink dirs if not requested otherwise. Android has a number
+				// of symlinks creating infinite source trees which would otherwise get
+				// us in an infinite loop.
+				// TODO(b/197349722): Revisit this once symlink loops are banned in the
+				// source tree.
+				if f.cacheMetadata.Config.FollowSymlinks {
+					subdirs = append(subdirs, child.Name())
+				}
 			} else {
 				// We do have to support symlink files because the link name might be
 				// different than the target name

finder/finder_test.go

@@ -90,6 +90,7 @@ func runSimpleTest(t *testing.T, existentPaths []string, expectedMatches []strin
 		CacheParams{
 			"/cwd",
 			[]string{root},
+			false,
 			nil,
 			nil,
 			[]string{"findme.txt", "skipme.txt"},
@@ -121,6 +122,7 @@ func runTestWithSuffixes(t *testing.T, existentPaths []string, expectedMatches [
 		CacheParams{
 			"/cwd",
 			[]string{root},
+			false,
 			nil,
 			nil,
 			[]string{"findme.txt", "skipme.txt"},