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

[Eli Friedman via llvm-commits]

================
@@ -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
----------------
efriedma-quic wrote:

Using try_run tends to lead to strange results with cross-compiling; do we really need it here?  _Float128/__float128 should always mean what we want it to, and we can use `__LDBL_MANT_DIG__` for long double.

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