Some MLIR libraries are intended to be dlopen-ed, but currently all MLIR
libraries link against LLVMSupport. After the recent PCH introduction,
this causes these libraries to implicitly use the LLVMSupport PCH, which
results in the definition of llvm::*ABIBreakingChecks, which results in
a ODR violation when loaded with dlopen.
Conceptually, libraries that are designed to be dlopen-ed should not
simply link against LLVM libraries in non-dylib builds for this reason.
(This apparently was a problem before with mlir_apfloat_wrappers.)
To fix builds, remove LLVMSupport from runtime libraries that don't need
it and, as a workaround, disable PCH for libraries that are in a weird
state (use LLVMSupport but happen to not export symbols currently).
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