[TargetParser] Disallow Global Constructors

[lenary]

Summary

Global Constructors are disallowed in the Support library. The
TargetParser library is likely to go along with the Support library in
most uses, because it contains llvm::Triple, so lets pre-emptively add
the same rule, rather than getting caught out later.

[lenary]

Tagging reviewers. @mehdi_amini I added you as you did the same change to the Support library in rG402461beb051b6a5c158f1e36d8e2c2b676e8804.

[tmatheson]

@mehdi_amini can you explain what the original reasoning behind this restriction was for Support, and whether we need to make the same restriction for TargetParser now that it's split out?

[dblaikie]

In D145249#4167428, @tmatheson wrote:

@mehdi_amini can you explain what the original reasoning behind this restriction was for Support, and whether we need to make the same restriction for TargetParser now that it's split out?

Global ctors/dtors are discouraged/disallowed by the LLVM style guide in general: https://llvm.org/docs/CodingStandards.html#do-not-use-static-constructors

So basically, I think, any library that can have the warning/error enabled should - to ensure we conform to this style rule, and don't regress.

[mehdi_amini]

My personal motivation was to be able to ship MLIR without any static ctor.
Fixing all of LLVM seemed too much, but MLIR (when not needing the JIT) does only depends on libSupport, which was very much doable (in particular a key piece was that the command line parsing entry point is part of the lib, so we can lazy load libSupport options)

llvm/lib/TargetParser/CMakeLists.txt

+# Ensure that libLLVMTargetParser does not carry any static global initializer.
+# ManagedStatic can be used to enable lazy-initialization of globals.
+#
+# HAS_WERROR_GLOBAL_CTORS and LLVM_HAS_NOGLOBAL_CTOR_MUTEX should have been set
+# by llvm/lib/Support/CMakeLists.txt (which provides the required Support
+# dependency).
+if (HAS_WERROR_GLOBAL_CTORS AND NOT LLVM_HAS_NOGLOBAL_CTOR_MUTEX)
+  SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=global-constructors")
+endif()
+
 add_llvm_component_library(LLVMTargetParser
   AArch64TargetParser.cpp
   ARMTargetParserCommon.cpp
   ARMTargetParser.cpp
   CSKYTargetParser.cpp
   Host.cpp
   LoongArchTargetParser.cpp
   RISCVTargetParser.cpp