[llvm] [llvm-c] Deprecate functions working on the global context (PR #163979)

[Alexander Richardson via llvm-commits]

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@@ -581,6 +581,9 @@ LLVM_C_ABI LLVMContextRef LLVMContextCreate(void);
 
 /**
  * Obtain the global context instance.
+ *
+ * Use of the global context is deprecated. Use LLVMContextCreate() and
+ * context-aware functions instead.
  */
 LLVM_C_ABI LLVMContextRef LLVMGetGlobalContext(void);
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arichardson wrote:

Could we conditionally deprecate this based on a `#ifndef BUILDING_LLVM_C_API` or similar macro that is defined in the one source file that needs to use it? 

https://github.com/llvm/llvm-project/pull/163979