[mlir][NFC] Rename StandardToLLVM to FuncToLLVM

The current StandardToLLVM conversion patterns only really handle
the Func dialect. The pass itself adds patterns for Arithmetic/CFToLLVM, but
those should be/will be split out in a followup. This commit focuses solely
on being an NFC rename.

Aside from the directory change, the pattern and pass creation API have been renamed:
 * populateStdToLLVMFuncOpConversionPattern -> populateFuncToLLVMFuncOpConversionPattern
 * populateStdToLLVMConversionPatterns -> populateFuncToLLVMConversionPatterns
 * createLowerToLLVMPass -> createConvertFuncToLLVMPass

Differential Revision: https://reviews.llvm.org/D120778