[clang] Enable C++11-style attributes in all language modes

[clang] Enable C++11-style attributes in all language modes

This also ignores and deprecates the `-fdouble-square-bracket-attributes` command line flag, which seems to not be used anywhere. At least a code search exclusively found mentions of it in documentation: https://sourcegraph.com/search?q=context:global+-fdouble-square-bracket-attributes+-file:clang/*+-file:test/Sema/*+-file:test/Parser/*+-file:test/AST/*+-file:test/Preprocessor/*+-file:test/Misc/*+archived:yes&patternType=standard&sm=0&groupBy=repo

RFC: https://discourse.llvm.org/t/rfc-enable-c-11-c2x-attributes-in-all-standard-modes-as-an-extension-and-remove-fdouble-square-bracket-attributes

This enables `[[]]` attributes in all C and C++ language modes without warning by default. `-Wc++-extensions` does warn. GCC has enabled this extension in all C modes since GCC 10.

Reviewed By: aaron.ballman, MaskRay

Spies: #clang-vendors, beanz, JDevlieghere, Michael137, MaskRay, sstefan1, jplehr, cfe-commits, lldb-commits, dmgreen, jdoerfert, wenlei, wlei

Differential Revision: https://reviews.llvm.org/D151683