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

[beanz]

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.

[zturner]

Seems fine to me, I know a lot of peoples' standard workflows involves putting the build directory in the root anyway.

One thing I can't tell by looking at the file paths - Is this in the LLDB directory or the LLVM directory? If it's in the LLDB directory then do we need it at the top level LLVM directory too? Although since it seems likely we'll eventually end up on mono-repo, maybe we need it in all project directories.

[beanz]

This is for LLVM. Will land shortly.

[MatzeB]

I usually put my personal ignore patterns like build directories into .git/info/exclude. Though I wouldn't object this commit either, it shouldn't hurt anyone.

llvm/trunk/.gitignore

 # Byte compiled python modules.
 *.pyc
 # vim swap files
 .*.sw?
 .sw?
 #OS X specific files.
 .DS_store
 
+# Nested build directory
+/build
+
 #==============================================================================#
 # Explicit files to ignore (only matches one).
 #==============================================================================#
 # Various tag programs
 /tags
 /TAGS
 /GPATH
 /GRTAGS