[clang] Reland: "[Frontend][PCH]-Add support for ignoring PCH options (-ignore-pch). (#142409)" (PR #143614)

[Fangrui Song via cfe-commits]

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@@ -0,0 +1,19 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+
+// Create PCH without -ignore-pch.
+// RUN: %clang -x c++-header %S/../Modules/Inputs/codegen-flags/foo.h -### 2>&1 | FileCheck %s -check-prefix=CHECK-EMIT-PCH
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MaskRay wrote:

`%S/../` references auxiliary file from a different test directory. We typically avoid doing this,,, though Index/ and Headers/ seem to have violations.

https://github.com/llvm/llvm-project/pull/143614