llvm dev - May 2021 - [DebugInfo] FP Constants representation

[AMD Public Use]

Hello Everyone,

I’ve started investigating representation of named FP constants in
`DIExpression`. A similar approach has been
tried and deployed in https://reviews.llvm.org/D99273 for named **integer**
constants.

I'm trying to represent named FP constants as:
```
!4 = !DIGlobalVariableExpression(var: !5, expr: (DW_OP_consts, **2.888000e+03**,
DW_OP_stack_value))
!5 = distinct !DIGlobalVariable(name: "real_constant",  scope: !2,
file: !3, line: 3, type: !9, isLocal: false, isDefinition: true)

Which should materialize in dwarf as:
    DW_TAG_variable 
         DW_AT_name "real_constant")
          DW_AT_type  "const real(kind=4)")
          DW_AT_const_value [DW_FORM_block1]    (<0x04> 00 80 34 45 )
```

While trying this I got stumbled upon `DIExpression` limitation(Or it could be a
design choice also ?). It seems that `DIExpression`  only accepts
`uint64_t` as its potential elements thereby leaving no room for things like FP
constants to be inside.
Digging in further, I realized that extending `DIExpression` for FP constants is
non-trivial/maybe breaking also ?

I need to have your/community thoughts on this approach or some other potential
approach 😊

**NOTE**: Named constants are extensively used in FORTRAN, but having this
support should benefit other
potential languages/FE also.

Thanks a lot!
Sourabh.