[clang-format] Support block indenting array/struct list initializers

C89 and C99 list initializers are treated differently than Cpp11 braced
initializers. This patch identifies the C array/struct initializer lists by
finding the preceding equal sign before a left brace, and applies formatting
rules for BracketAlignmentStyle.BlockIndent to those list initializers.

Fixes #57878.

Differential Revision: https://reviews.llvm.org/D153205