[TF] Rename -DCOMPILING_TENSORFLOW_STDLIB_MODULE CMake flag.

`-DCOMPILING_TENSORFLOW_MODULE` has been renamed to
`-DCOMPILING_TENSORFLOW_STDLIB_MODULE` for clarity.

"STDLIB" emphasizes that the flag is relevant for building the Swift stdlib.

Author: dan-zheng

stdlib/public/TensorFlow/CMakeLists.txt

@@ -29,7 +29,7 @@ list(APPEND swift_stdlib_compile_flags "-force-single-frontend-invocation")
 # FIXME(SR-7972): Some tests fail when TensorFlow is optimized.
 # list(APPEND swift_stdlib_compile_flags "-O" "-whole-module-optimization")
 list(APPEND swift_stdlib_compile_flags "-Onone")
-list(APPEND swift_stdlib_compile_flags "-DCOMPILING_TENSORFLOW_MODULE")
+list(APPEND swift_stdlib_compile_flags "-DCOMPILING_TENSORFLOW_STDLIB_MODULE")
 
 set(SOURCES "")