[flang] Outline operator<< for CharBlock. (flang-compiler/f18#916)

This fixes an issue where the Dump function definitions in dump.cc
were relying on the forward declaration of operator<< for CharBlock
which was marked inline and only present in char-block.h.

This is not allowed under section 6.2.10 of the C++17 standard, and
caused a compilation failure when building with clang 9 as this was
inlining every use of the function and therefore not generating an
outlined definition for linking.

Original-commit: flang-compiler/f18@3ad75d123b666a5081540f2afedbedde930b2518
Reviewed-on: https://github.com/flang-compiler/f18/pull/916