[clang-format] Add `SpacesInAngles: Leave` option to keep spacing inside angle brackets as is.

A need for such an option came up in a few libc++ reviews. That's because libc++ has both code in C++03 and newer standards.
Currently, it uses `Standard: C++03` setting for clang-format, but this breaks e.g. u8"string" literals.
Also, angle brackets are the only place where C++03-specific formatting needs to be applied.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D101344