[libcxxabi] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles#134976

mstorsjo merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

mstorsjo:libcxxabi-demangle-ldblmstorsjo/llvm-project:libcxxabi-demangle-ldblCopy head branch name to clipboard