Add --build-id=fast as a synonym for --build-id.

When you omit an argument, most options fall back to their defaults.
For example, --color-diagnostics is a synonym for --color-diagnostics=auto.
We don't have a way to specify the default choice for --build-id, so we
can't describe --build-id (without an argument) in that way.
This patch adds "fast" for the default build-id choice.

Differential Revision: https://reviews.llvm.org/D43032

llvm-svn: 324502