Pre-Commit test: [Clang][Preprocessor] Enable macro expansion **only** in directives when parsing preprocessed sources

Author: jmmartinez

clang/test/Preprocessor/preprocess-pragma-cpp-output.c

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -E -x c %s | FileCheck %s
+// RUN: %clang_cc1 -x c -fsyntax-only %s -verify
+// expected-no-diagnostics
+// RUN: %clang_cc1 -x cpp-output -fsyntax-only -verify=fail %s
+
+// The preprocessor does not expand macro-identifiers in #pragma directives.
+// When we preprocess & parse the code, clang expands the macros in directives.
+// When we parse already preprocessed code, clang still has to expand the
+// macros in the directives.
+// This means that we're not always able to parse the preprocessor's output
+// without preserving the definitions (-dD).
+
+#define FACTOR 4
+
+void foo() {
+    // CHECK: #pragma unroll FACTOR
+    #pragma unroll FACTOR // fail-error{{use of undeclared identifier}}
+    for(;;) {
+    }
+    return;
+}