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
Merge pull request #3868 from 9rnsr/fix13281

Issue 13281 - Print type suffix of real/complex literals in pragma(msg) and error diagnostic