Fix a unit test input file (llvm#102567)

I forgot to update the version info in the SDKSettings file when I
updated it to the real version relevant to the test.

(cherry picked from commit 4c5ef66)

Author: ian-twilightcoder

clang/test/Driver/Inputs/MacOSX15.0.sdk/SDKSettings.json

@@ -1 +1 @@
-{"Version":"990.0", "MaximumDeploymentTarget": "99.0.99"}
+{"Version":"15.0", "MaximumDeploymentTarget": "15.0.99"}