[upstream] Allow vector_size applied to Boolean

[simoll]

Upstream support for the _Bool/bool element type with the vector_size(N) type attribute. We will use this to clean up the VE vector intrinsics and get rid of the v4i64/v8i64 mappings for the vector mask types in favor of v256i2/v512i1 only.

The patch is currently under review on the LLVM Phabricator and also tracked in this repo.

[simoll]

ext_vector_type variant merged (llvm/llvm-project commit 0aab344)