Reland "[flang] Avoid undefined behaviour when parsing format expressions (#147539)" (#148169)

This reverts commit e8e5d07767c444913f837dd35846a92fcf520eab.

This previously failed because the flang-rt build could not find the 
llvm header file. It passed on some machines but only because they
had globally installed copies of older llvm.

To fix this, I've copied the required routines from llvm into flang.

With the following justification:
* Flang can, and does, use llvm headers.
* Some Flang headers are also used in Flang-rt.
* Flang-rt cannot use llvm headers.
* Therefore any Flang header using in Flang-rt cannot use llvm headers
either.

To support that conclusion,
https://flang.llvm.org/docs/IORuntimeInternals.html
states:
"The Fortran I/O runtime support library is written in C++17, and uses
some C++17 standard library facilities, but it is intended to not have
any link-time dependences on the C++ runtime support library or any LLVM
libraries."

This talks about libraries but I assume it applies to llvm in general.

Nothing in flang/include/flang/Common, or flang/include/flang/Common
includes any llvm header, and I see some very similar headers there
that duplicate llvm functionality. Like float128.h.

I can only assume this means these files must remain free of
dependencies
on LLVM.

I have copied the two routines literally and put them in the
flang::common
namespace, for lack of a better place for them. So they don't clash with
something.

I have specialised the function to the 1 type flang needs, as it might
save a bit of compile time.