Ignore all files beginning with build in the top level (llvm#303)

- leading slash matches only in the top level directory
- asterisk allows matching build2, build-sharedlibs, or others

This is useful if you need more than one build directory for testing
with different cmake options.

.gitignore

@@ -1,4 +1,4 @@
-build
+/build*
 /docker_build
 .vscode
 #*