Introduce LLVM_HOT

[xbolva00]

Summary

From GCC docs (I found no docs for this attribute in LLVM docs; but LLVM understands hot/cold attribute too):
"The hot attribute on a function is used to inform the compiler that the function is a hot spot of the compiled program. The function is optimized more aggressively and on many targets it is placed into a special subsection of the text section so all hot functions appear close together, improving locality."

https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

We can mark Clang/LLVM hotspots with LLVM_HOT as follow-up.

[kristina]

I think that's best left to PGO, manually marking LLVM/Clang/etc. codebase with hotspots seems difficult and unreliable. I can't think of many places where this would actually be universally useful. If you can think of such cases then maybe this would be of some use but right now, I don't really see the point, sorry.

Also please add reviewers to your differentials when you make one, otherwise it's in the review queue with no assigned reviewers.

include/llvm/Support/Compiler.h

 #if __has_attribute(returns_nonnull) || LLVM_GNUC_PREREQ(4, 9, 0)
 #define LLVM_ATTRIBUTE_RETURNS_NONNULL __attribute__((returns_nonnull))
 #elif defined(_MSC_VER)
 #define LLVM_ATTRIBUTE_RETURNS_NONNULL _Ret_notnull_
 #else
 #define LLVM_ATTRIBUTE_RETURNS_NONNULL
 #endif
 
+#if __has_attribute(hot)
+#define LLVM_HOT __attribute__((__hot__))
+#else
+#define LLVM_HOT
+#endif
+
 /// \macro LLVM_ATTRIBUTE_RETURNS_NOALIAS Used to mark a function as returning a
 /// pointer that does not alias any other valid pointer.
 #ifdef __GNUC__
 #define LLVM_ATTRIBUTE_RETURNS_NOALIAS __attribute__((__malloc__))
 #elif defined(_MSC_VER)
 #define LLVM_ATTRIBUTE_RETURNS_NOALIAS __declspec(restrict)
 #else
 #define LLVM_ATTRIBUTE_RETURNS_NOALIAS