[llvm] Enable logf128 constant folding for hosts with 128bit long double (PR #96287)

[Eli Friedman via llvm-commits]

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@@ -271,6 +260,64 @@ if(C_SUPPORTS_WERROR_UNGUARDED_AVAILABILITY_NEW)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unguarded-availability-new")
 endif()
 
+function(logf128_test testname definition)
+  unset(LOGF128_TEST_RUN CACHE)
+  unset(LOGF128_TEST_COMPILE CACHE)
+  try_run(
+    LOGF128_TEST_RUN
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efriedma-quic wrote:

You only need to check `__LDBL_MANT_DIG__` if you're trying to use the type `long double`.  You can assume `__float128` and `_Float128` have the right format.

https://github.com/llvm/llvm-project/pull/96287