Speed up vendor snapshot header globs

The vendor snapshot header globs were repeated 9 times, once for each
possible header suffix.  Globbing is not a fast operation, so this
took 9.68s of CPU time in my AOSP builds.  Replace it instead with a
single glob, followed by filtering down to the desired header
extensions.

Test: m checkbuild
Change-Id: I9c8f8cdad8cbffc0a42217659a48dd9d676e4e4c

cc/library.go

@@ -449,23 +449,37 @@ func (l *libraryDecorator) collectHeadersForSnapshot(ctx android.ModuleContext)
 			}
 			continue
 		}
-		exts := headerExts
-		// Glob all files under this special directory, because of C++ headers.
-		if strings.HasPrefix(dir, "external/libcxx/include") {
-			exts = []string{""}
+		glob, err := ctx.GlobWithDeps(dir+"/**/*", nil)
+		if err != nil {
+			ctx.ModuleErrorf("glob failed: %#v", err)
+			return
 		}
-		for _, ext := range exts {
-			glob, err := ctx.GlobWithDeps(dir+"/**/*"+ext, nil)
-			if err != nil {
-				ctx.ModuleErrorf("glob failed: %#v", err)
-				return
-			}
-			for _, header := range glob {
-				if strings.HasSuffix(header, "/") {
+		isLibcxx := strings.HasPrefix(dir, "external/libcxx/include")
+		j := 0
+		for i, header := range glob {
+			if isLibcxx {
+				// Glob all files under this special directory, because of C++ headers with no
+				// extension.
+				if !strings.HasSuffix(header, "/") {
+					continue
+				}
+			} else {
+				// Filter out only the files with extensions that are headers.
+				found := false
+				for _, ext := range headerExts {
+					if strings.HasSuffix(header, ext) {
+						found = true
+						break
+					}
+				}
+				if !found {
 					continue
 				}
-				ret = append(ret, android.PathForSource(ctx, header))
 			}
+			if i != j {
+				glob[j] = glob[i]
+			}
+			j++
 		}
 	}