[clang] Ensure proper NULL macro definition for system include files. (PR #149176)

[Jamie Schmeiser via cfe-commits]

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+// There are at least 2 valid C null-pointer constants as defined
+// by the C language standard.
+// Test that the macro NULL is defined consistently for all platforms by
+// those headers that the C standard mandates a macro definition for NULL.
+
+// RUN: %clang %s -Dheader="<locale.h>" -E | tail -1 | FileCheck %s
+// RUN: %clang %s -Dheader="<stdio.h>" -E | tail -1 | FileCheck %s
+// RUN: %clang %s -Dheader="<stdlib.h>" -E | tail -1 | FileCheck %s
+// RUN: %clang %s -Dheader="<string.h>" -E | tail -1 | FileCheck %s
+// RUN: %clang %s -Dheader="<time.h>" -E | tail -1 | FileCheck %s
+// RUN: %clang %s -Dheader="<wchar.h>" -E | tail -1 | FileCheck %s
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jamieschmeiser wrote:

How can that happen?  According to https://clang.llvm.org/docs/CrossCompilation.html:  `If you don’t specify the target, CPU names won’t match (since Clang assumes the host triple), and the compilation will go ahead, creating code for the host platform`. There is no target specified so it will not be cross-compiling.

Is there something that you would suggest to alleviate your concerns?

https://github.com/llvm/llvm-project/pull/149176