[llvm] [CI] Add flang-rt to pre-merge configuration (PR #156039)

[Aiden Grossman via llvm-commits]

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@@ -80,7 +81,8 @@
     "clang-tools-extra": {"libc"},
     "libc": {"libc"},
     "compiler-rt": {"compiler-rt"},
-    ".ci": {"compiler-rt", "libc"},
+    "flang-rt": {"flang-rt"},
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boomanaiden154 wrote:

We should probably test `flang-rt` on `flang` changes.

https://github.com/llvm/llvm-project/pull/156039