[HLSL][RootSignature] Implement `ResourceRange` as an `IntervalMap` (#140957)

A resource range consists of a closed interval, `[a;b]`, denoting which
shader registers it is bound to.

For instance:
 - `CBV(b1)`  corresponds to the resource range of `[1;1]`
 - `CBV(b0, numDescriptors = 3)` likewise to `[0;2]`

We want to provide an error diagnostic when there is an overlap in the
required registers (an overlap in the resource ranges).

The goal of this pr is to implement a structure to model a set of
resource ranges and provide an api to detect any overlap over a set of
resource ranges.

`ResourceRange` models this by implementing an `IntervalMap` to denote a
mapping from an interval of registers back to a resource range. It
allows for a new `ResourceRange` to be added to the mapping and it will
report if and what the first overlap is.

For the context of how this will be used in validation of a
`RootSignatureDecl` please see the proceeding pull request here:
https://github.com/llvm/llvm-project/pull/140962.

- Implements `ResourceRange` as an `IntervalMap`
- Adds unit testing of the various `insert` scenarios

Note: it was also considered to implement this as an `IntervalTree`,
this would allow reporting of a diagnostic for each overlap that is
encountered, as opposed to just the first. However, error generation of
just reporting the first error is already rather verbose, and adding the
additional diagnostics only made this worse.

Part 1 of https://github.com/llvm/llvm-project/issues/129942