[TooltipOverlay] Add border radius to tooltip overlay (#8093)

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### WHY are these changes introduced?

<!--
  Context about the problem that’s being addressed.
-->

Tooltip overlays don't currently have a border radius applied to them.
This is more noticeable when the `borderRadius` prop passed to `Tooltip`
is a higher number.


#### Before

<img width="429" alt="Screenshot 2023-01-19 at 12 01 19"
src="https://user-images.githubusercontent.com/6731221/213437869-632b1210-aac5-4524-989c-34196b2ea483.png">

#### After

<img width="433" alt="Screenshot 2023-01-19 at 12 00 48"
src="https://user-images.githubusercontent.com/6731221/213437769-e5fc13b5-a9f4-401c-a15b-781cc4c922f3.png">


### WHAT is this pull request doing?

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This PR uses the `pc-tooltip-border-radius` variable (specified via the
inline style on `TooltipOverlay`'s container div) to apply border radius
in the `.TooltipOverlay` class style.

### 🎩 checklist

[Tophat on
Chromatic](https://5d559397bae39100201eedc1-xvkjbwonew.chromatic.com/?path=/story/all-components-tooltip--border-radius)

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [x] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: highfieldjames

.changeset/bright-melons-obey.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Added `borderRadius` style to `TooltipOverlay`

polaris-react/src/components/Tooltip/components/TooltipOverlay/TooltipOverlay.scss

@@ -7,6 +7,8 @@
   margin: var(--pc-tooltip-overlay-offset) var(--p-space-4) var(--p-space-4);
   opacity: 1;
   box-shadow: var(--p-shadow-popover);
+  // stylelint-disable-next-line -- Polaris component custom properties
+  border-radius: var(--pc-tooltip-border-radius);
   pointer-events: none;
   will-change: opacity, left, top;
   transition: opacity var(--p-duration-200) var(--p-ease-in)