[.gitignore] Update .gitignore to ignore a nested build directory

Summary:
A number of tools and common workflows include putting a build directory inside the source checkout under the folder "build". Adding this to .gitignore seems useful.

As an example, the CMake Tools plugin for VSCode does this.

Reviewers: chandlerc, echristo, zturner

Reviewed By: zturner

Subscribers: MatzeB, mehdi_amini, llvm-commits, jgosnell

Differential Revision: https://reviews.llvm.org/D30346

llvm-svn: 296188