remove Demangle/StringView.h

Now that we've converted libcxxabi and llvm Demangle to use
std::string_view, this code no longer has any users. Bye bye!

Reviewed By: #libc_abi, phosek, MaskRay

Differential Revision: https://reviews.llvm.org/D148387

Author: nickdesaulniers

clang/docs/tools/clang-formatted-files.txt

@@ -3102,7 +3102,6 @@ libc/src/__support/CPP/Bit.h
 libc/src/__support/CPP/Bitset.h
 libc/src/__support/CPP/Functional.h
 libc/src/__support/CPP/Limits.h
-libc/src/__support/CPP/StringView.h
 libc/src/__support/CPP/Utility.h
 libc/src/__support/CPP/vector.h
 libc/src/__support/File/file.cpp
@@ -3451,7 +3450,6 @@ libcxx/src/ryu/d2fixed.cpp
 libcxx/src/ryu/d2s.cpp
 libcxx/src/ryu/f2s.cpp
 libcxxabi/src/cxa_guard_impl.h
-libcxxabi/src/demangle/StringView.h
 libcxxabi/src/demangle/Utility.h
 libunwind/src/cet_unwind.h
 lld/COFF/CallGraphSort.cpp
@@ -5250,7 +5248,6 @@ llvm/include/llvm/Debuginfod/Debuginfod.h
 llvm/include/llvm/Debuginfod/DIFetcher.h
 llvm/include/llvm/Debuginfod/HTTPClient.h
 llvm/include/llvm/Demangle/Demangle.h
-llvm/include/llvm/Demangle/StringView.h
 llvm/include/llvm/Demangle/StringViewExtras.h
 llvm/include/llvm/Demangle/Utility.h
 llvm/include/llvm/DWARFLinker/DWARFLinker.h

libcxxabi/src/demangle/StringView.h

@@ -6,7 +6,7 @@
 set -e
 
 cd $(dirname $0)
-HDRS="ItaniumDemangle.h ItaniumNodes.def StringView.h StringViewExtras.h Utility.h"
+HDRS="ItaniumDemangle.h ItaniumNodes.def StringViewExtras.h Utility.h"
 LLVM_DEMANGLE_DIR=$1
 
 if [[ -z "$LLVM_DEMANGLE_DIR" ]]; then

libcxxabi/src/demangle/cp-to-llvm.sh

@@ -10,5 +10,4 @@ add_llvm_unittest(DemangleTests
   OutputBufferTest.cpp
   PartialDemangleTest.cpp
   RustDemangleTest.cpp
-  StringViewTest.cpp
 )