[CAS] Make CAS plugin options to be cache invariant

Do not add CAS configurations into cache key computation. This ensure if
the cache keys are identical if two functionally equivalent plugins are
used. This is safe to do because if any of the CAS configurations
changes how the cache key is computed, they are going to be directly
reflected in the cache key without adding its configuration into the
key.

rdar://137091843

Author: cachemeifyoucan

include/swift/Option/Options.td

@@ -1999,11 +1999,11 @@ def cas_path: Separate<["-"], "cas-path">,
   HelpText<"Path to CAS">, MetaVarName<"<path>">;
 
 def cas_plugin_path: Separate<["-"], "cas-plugin-path">,
-  Flags<[FrontendOption, NewDriverOnlyOption]>,
+  Flags<[FrontendOption, NewDriverOnlyOption, CacheInvariant]>,
   HelpText<"Path to CAS Plugin">, MetaVarName<"<path>">;
 
 def cas_plugin_option: Separate<["-"], "cas-plugin-option">,
-  Flags<[FrontendOption, NewDriverOnlyOption]>,
+  Flags<[FrontendOption, NewDriverOnlyOption, CacheInvariant]>,
   HelpText<"Option pass to CAS Plugin">, MetaVarName<"<name>=<option>">;
 
 def clang_scanner_module_cache_path : Separate<["-"], "clang-scanner-module-cache-path">,

test/CAS/cache_key_compute.swift

@@ -50,6 +50,14 @@
 // RUN: diff %t5.casid %t6.casid
 // RUN: not diff %t5.casid %t7.casid
 
+/// Check switching CAS plugin path.
+// RUN: %cache-tool -cas-path %t/cas -cas-plugin-path %llvm_libs_dir/libCASPluginTest%llvm_plugin_ext -cache-tool-action print-base-key -- \
+// RUN:   %target-swift-frontend -cache-compile-job %t/a.swift -c @%t/MyApp.cmd -cas-path %t/cas -cas-plugin-path %llvm_libs_dir/libCASPluginTest%llvm_plugin_ext > %t8.casid
+// RUN: ln -s %llvm_libs_dir/libCASPluginTest%llvm_plugin_ext %t/libCASPluginTest%llvm_plugin_ext
+// RUN: %cache-tool -cas-path %t/cas -cas-plugin-path %t/libCASPluginTest%llvm_plugin_ext -cache-tool-action print-base-key -- \
+// RUN:   %target-swift-frontend -cache-compile-job %t/a.swift -c @%t/MyApp.cmd -cas-path %t/cas -cas-plugin-path %t/libCASPluginTest%llvm_plugin_ext > %t9.casid
+// RUN: diff %t8.casid %t9.casid
+
 /// Test output keys.
 // RUN: %cache-tool -cas-path %t/cas -cache-tool-action print-output-keys -- \
 // RUN:   %target-swift-frontend -cache-compile-job %t/a.swift -emit-module -c -emit-dependencies \