Fix invalid substitution in IRGen/static-library.swift (#79607)

%swiftc_driver_plain is initialized from the plain swiftc path in config.swiftc, while %swiftc_driver gets extras like SDKROOT, stdlib-rpath and custom options. There is no plain pendant for the 'target-' prefixed version, but the regular one seems fine.

Author: weliveindetail

test/IRGen/static-library.swift

@@ -1,7 +1,7 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swiftc_driver_plain -DLIBRARY -module-name Library -emit-library -static -autolink-force-load -module-link-name Library %s -o %t/library.lib -emit-module-path %t
-// RUN: %target-swiftc_driver_plain -DLIBRARY -module-name Library -emit-library -static -autolink-force-load -module-link-name Library %s -S -emit-ir -o - | %FileCheck -check-prefix CHECK-LIBRARY %s
-// RUN: %target-swiftc_driver_plain -I %t -emit-library -S -emit-ir -o - %s | %FileCheck -check-prefix CHECK-EMBEDDING %s
+// RUN: %target-swiftc_driver -DLIBRARY -module-name Library -emit-library -static -autolink-force-load -module-link-name Library %s -o %t/library.lib -emit-module-path %t
+// RUN: %target-swiftc_driver -DLIBRARY -module-name Library -emit-library -static -autolink-force-load -module-link-name Library %s -S -emit-ir -o - | %FileCheck -check-prefix CHECK-LIBRARY %s
+// RUN: %target-swiftc_driver -I %t -emit-library -S -emit-ir -o - %s | %FileCheck -check-prefix CHECK-EMBEDDING %s
 
 // REQUIRES: OS=windows-msvc