Collapse target union types before narrowing

[jeskew]

Resolves #10098

When narrowing types, we should collapse unions as an early first step. This is primarily done to enhance constraint checks and error messaging

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test Results (linux-x64)

       33 files  ±0         33 suites  ±0   36m 17s ⏱️ + 3m 10s
10 066 tests +1  10 066 ✔️ +1  0 💤 ±0  0 ❌ ±0 
12 282 runs  +1  12 282 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 77dab8c. ± Comparison against base commit 6f98adc.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results (win-x64)

       33 files  ±0         33 suites  ±0   29m 50s ⏱️ - 2m 57s
10 078 tests +1  10 078 ✔️ +1  0 💤 ±0  0 ❌ ±0 
12 293 runs  +1  12 293 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 77dab8c. ± Comparison against base commit 6f98adc.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results (osx-x64)

       33 files  ±0         33 suites  ±0   1h 36m 14s ⏱️ + 10m 4s
10 070 tests +1  10 070 ✔️ +1  0 💤 ±0  0 ❌ ±0 
12 286 runs  +1  12 286 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 77dab8c. ± Comparison against base commit 6f98adc.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results (linux-musl-x64)

       33 files  ±0         33 suites  ±0   28m 25s ⏱️ +28s
10 066 tests +1  10 066 ✔️ +1  0 💤 ±0  0 ❌ ±0 
12 282 runs  +1  12 282 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit 77dab8c. ± Comparison against base commit 6f98adc.

♻️ This comment has been updated with latest results.