[clang] [compiler-rt] [llvm] [UBSan] Add -fsanitize=uninitialized-read check (ISO C 6.3.2.1p2) (PR #207529)

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<!--LLVM CODE FORMAT COMMENT: {clang-format}-->


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You can test this locally with the following command:
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``````````bash
git-clang-format --diff origin/main HEAD --extensions cpp,h,c -- compiler-rt/test/msan/local_uninit_read.c clang/include/clang/Driver/SanitizerArgs.h clang/lib/CodeGen/BackendUtil.cpp clang/lib/CodeGen/CGExprScalar.cpp clang/lib/CodeGen/CodeGenFunction.h clang/lib/Driver/SanitizerArgs.cpp llvm/include/llvm/IR/Instruction.h llvm/include/llvm/Transforms/Instrumentation/MemorySanitizer.h llvm/lib/IR/Metadata.cpp llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp --diff_from_common_commit
``````````

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The reproduction instructions above might return results for more than one PR
in a stack if you are using a stacked PR workflow. You can limit the results by
changing `origin/main` to the base branch/commit you want to compare against.
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View the diff from clang-format here.
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``````````diff
diff --git a/compiler-rt/test/msan/local_uninit_read.c b/compiler-rt/test/msan/local_uninit_read.c
index 484e7d5b9..f3286d6b9 100644
--- a/compiler-rt/test/msan/local_uninit_read.c
+++ b/compiler-rt/test/msan/local_uninit_read.c
@@ -72,16 +72,18 @@ int main() {
 #elif defined(ASM_INIT)
 int main() {
   int x;
-#if defined(__x86_64__) || defined(__i386__)
+#  if defined(__x86_64__) || defined(__i386__)
   __asm__("movl $0, %0" : "=r"(x));
-#else
+#  else
   x = 0;
-#endif
+#  endif
   x;
   return 0;
 }
 #elif defined(AGGREGATE)
-struct S { int a; };
+struct S {
+  int a;
+};
 int main() {
   struct S s;
   s.a;

``````````

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https://github.com/llvm/llvm-project/pull/207529