tsan: move shadow declaration into a separate header file (NFC)

tsan_rtl.h is very huge and contains too many things.
Move FastState and Shadow types into a new tsan_shadow.h file.
This also allows to use FastState/Shadow in other header files
without creating circular dependencies (which most likely will
happen today).

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D110272