Cherry-pick: ICU-21449 Infinite loop can occur with locale IDs containing RES_PATH_SEPARATOR

[jefgen]

This change cherry-picks the upstream fix for the issue of an infinite loop when calling various APIs with locale IDs that contain RES_PATH_SEPARATOR.

Upstream ticket:
https://unicode-org.atlassian.net/browse/ICU-21449

Upstream PR:
unicode-org/icu#1549

Summary

PR Checklist

• I have verified that my change is specific to this fork and cannot be made upstream.
• I am making a maintenance related change.
• I am making a change that is related to usage internal to Microsoft.
• I am making a change that is related to the Windows OS build of ICU.
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Detailed Description

From the upstream PR:

When calling APIs like uloc_getDisplayLanguage, if the input locale ID has / (the RES_PATH_SEPARATOR value) then the resource look-up code ends up in an infinite loop.

For example, the following code will never return:

UErrorCode status = U_ZERO_ERROR;
const char* localeNameTemp = "/"; // RES_PATH_SEPARATOR
uloc_getDisplayLanguage(localeNameTemp, localeNameTemp, NULL, 0, &status);

The do {...} while loop in ures_getByKeyWithFallback calls res_findResource, but it never checks to see if the returned resource is RES_BOGUS, meaning that for some paths it will never terminate. (Thanks to am11 for doing the initial investigation into this issue.)

This PR also includes a test case for the issue. Note that there is nothing really "tested" by the test case, as the test is to simply ensure that calling the API doesn't cause a hang due to the infinite loop.