[clang-format][TableGen] Don't add spaces around items in square braces.

Summary:
clang-formatting wants to add spaces around items in square braces, e.g. [1, 2] -> [ 1, 2 ]. Based on a quick check [1], it seems like most cases are using the [1, 2] format, so make that the consistent one.

[1] in llvm `.td` files, the regex `\[[^ ]` (bracket followed by not-a-space) shows up ~400 times, but `\[\s[^ ]` (bracket followed by one space and one not-a-space) shows up ~40 times => ~90% uses this format.

Reviewers: djasper, krasimir, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: MyDeveloperDay, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D55964

llvm-svn: 355158