[clang] [flang] [Flang] Add -ffast-real-mod back for further control of MOD optimizations (PR #167118)

[Tarun Prabhu via cfe-commits]

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@@ -1,5 +1,7 @@
+! RUN: %flang -ffast-real-mod -### -c %s 2>&1 | FileCheck %s -check-prefix CHECK-FAST-REAL-MOD
 ! RUN: %flang -fno-fast-real-mod -### -c %s 2>&1 | FileCheck %s -check-prefix CHECK-NO-FAST-REAL-MOD
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tarunprabhu wrote:

It may be a good idea to add tests that check what happens when both `-ffast-real-mod` and `-fno-fast-real-mod` appear on the command line. The last to appear should "win".

Since these are related to `ffast-math`, it may be good to check that they work as expected in the presence of `-ffast-math`

https://github.com/llvm/llvm-project/pull/167118