Remove obsolete functions from llvm_ext.cc

[HertzDevil]

Removes the functions that are known to be present since LLVM 8. No public interfaces are affected.

Here is a breakdown of the remaining functions in llvm_ext.cc:

• DIBuilder functions
  All of them exist on LLVM 8, but LLVM claims that the C interface is still experimental, and I haven't verified that the API remains unchanged in all versions up to 15. Also fun LLVMDisposeDIBuilder(builder : DIBuilderRef) was missing so it probably doesn't make sense for LLVM::DIBuilder to be a struct anymore. EDIT: LLVMExtDIBuilderCreateEnumerator did not exist in LLVM 8.0.
• LLVMExtSetCurrentDebugLocation
  This can be replaced by LLVMSetCurrentDebugLocation in LLVM 8, and LLVMSetCurrentDebugLocation2 in LLVM 9+. The first one accepts a non-nil LibLLVM::ValueRef, the second a nilable LibLLVM::MetadataRef (passing nil clears the debug location). LLVMDIBuilderCreateDebugLocation can create the corresponding LibLLVM::MetadataRef, but it also requires a LibLLVM::ContextRef. So far I haven't found a good way to pass the context around without causing any breaking changes in our "public" LLVM API. EDIT: I think this is just @llvm_module.context
• LLVMExtWriteBitcodeWithSummaryToFile
  5 arguments are passed to llvm::WriteBitcodeToFile here, whereas the LLVM C API can only accept the first 2. Two of the extra arguments are only relevant for the now removed ThinLTO support. I have no idea if the other extra argument, ShouldPreserveUseListOrder, must be true; if not then this function can be replaced by LLVMWriteBitcodeToFile.
• LLVMExtBuildOperandBundleDef, LLVMExtBuildCall2, LLVMExtBuildInvoke2
  The LLVM C API still doesn't expose functions that deal with operand bundles, which are apparently needed for exception handling. There is a stale LLVM revision for this.
• LLVMExtTargetMachineEnableGlobalIsel, LLVMExtCreateMCJITCompilerForModule
  The LLVM C API still doesn't expose functions that deal with global instruction selection. See also Disable LLVM Global Isel #9401.