[.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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296188
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