[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