[AST] Refine the condition for element-dependent array fillers

This patch fixes clang to not consider braced initializers for
aggregate elements of arrays to be potentially dependent on the
indices of the initialized elements. Resolves bug 18978:
initialize a large static array = clang oom?
https://bugs.llvm.org/show_bug.cgi?id=18978

Differential Revision: https://reviews.llvm.org/D43187

llvm-svn: 325120