Fix Issue 12745 - [Ddoc] Underscore is removed from numbers
Test 2 underscores
fix Issue 13281 - Print type suffix of real/complex literals in pragma(msg) and error diagnostic

Use PrettyPrintVisitor::floatToBuffer from RealExp and ComplexExp::toChars().
The change in diag9375 is intended to print exact value.
[Refactoring] Remove unnecessary Expression::toChars() overridings
fix Issue 13289 - wchar and dchar C++ mangling is incorrect