Use full path to clang++ when expanding %clang in lit tests (swiftlang#39180)

Author: kubamracek

test/lit.cfg

@@ -312,6 +312,7 @@ config.swift_indent = inferSwiftBinary('swift-indent')
 config.swift_symbolgraph_extract = inferSwiftBinary('swift-symbolgraph-extract')
 config.swift_api_extract = inferSwiftBinary('swift-api-extract')
 config.clang = inferSwiftBinary('clang')
+config.clangxx = inferSwiftBinary('clang++')
 config.llvm_link = inferSwiftBinary('llvm-link')
 config.swift_llvm_opt = inferSwiftBinary('swift-llvm-opt')
 config.llvm_profdata = inferSwiftBinary('llvm-profdata')
@@ -1153,8 +1154,8 @@ if run_vendor == 'apple':
          "-Xlinker -rpath -Xlinker /usr/lib/swift %s ")%
         (xcrun_prefix, config.swiftc, target_options, config.swift_driver_test_options))
     config.target_clang = (
-        "%s clang++ %s" %
-        (xcrun_prefix, config.target_cc_options))
+        "%s %s %s" %
+        (xcrun_prefix, config.clangxx, config.target_cc_options))
 
     config.target_build_swift_dylib = (
         "%s -parse-as-library -emit-library -o '\\1' "