[clang] [Clang] A lone `[` does not an attribute make (PR #147306)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> In some tentative parses, we would always consider `[` as the start of an attribute - only `[[` should be.
> 
> Fixes #63880

It is the start of an attribute: Microsoft-style attributes are single bracket rather than double bracket.

https://github.com/llvm/llvm-project/pull/147306