[flang] Support known constant lengths in DynamicType
The constexpr-capable class evaluate::DynamicType represented
CHARACTER length only with a nullable pointer into the declared
parameters of types in the symbol table, which works fine for
anything with a declaration but turns out to not suffice to
describe the results of the ACHAR() and CHAR() intrinsic
functions. So extend DynamicType to also accommodate known
constant CHARACTER lengths, too; use them for ACHAR & CHAR;
clean up several use sites and fix regressions found in test.
Differential Revision: https://reviews.llvm.org/D103571
13 files changed: