Revert "Cache memory buffer's name length"

This reverts commit 18244321.

The change triggers an assertion when building clang on macOS:

FAILED: tools/clang/include/clang/Tooling/Syntax/Nodes.inc
cd /Users/florianhahn/projects/llvm-project/builds/release-with-assertions/ccache-stage1 && /Users/florianhahn/projects/llvm-project/builds/release-with-assertions/ccache-stage1/bin/clang-tblgen -gen-clang-syntax-node-list -I /Users/florianhahn/projects/llvm-project/clang/include/clang/Tooling/Syntax -I/Users/florianhahn/projects/llvm-project/clang/include -I/Users/florianhahn/projects/llvm-project/builds/release-with-assertions/ccache-stage1/tools/clang/include -I/Users/florianhahn/projects/llvm-project/builds/release-with-assertions/ccache-stage1/include -I/Users/florianhahn/projects/llvm-project/llvm/include /Users/florianhahn/projects/llvm-project/clang/include/clang/Tooling/Syntax/Nodes.td --write-if-changed -o tools/clang/include/clang/Tooling/Syntax/Nodes.inc -d tools/clang/include/clang/Tooling/Syntax/Nodes.inc.d
Assertion failed: ((!RequiresNullTerminator || BufEnd[0] == 0) && "Buffer is not null terminated!"), function init, file MemoryBuffer.cpp, line 52.
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  clang-tblgen             0x000000010466b68c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  clang-tblgen             0x000000010466a808 llvm::sys::RunSignalHandlers() + 64
2  clang-tblgen             0x000000010466bd60 SignalHandler(int) + 344
3  libsystem_platform.dylib 0x00000001877a92a4 _sigtramp + 56
4  libsystem_pthread.dylib  0x000000018777acec pthread_kill + 288
5  libsystem_c.dylib        0x00000001876b42c8 abort + 180
6  libsystem_c.dylib        0x00000001876b3620 err + 0
7  clang-tblgen             0x00000001046ce150 llvm::ErrorOr<std::__1::unique_ptr<llvm::MemoryBuffer, std::__1::default_delete<llvm::MemoryBuffer>>> getOpenFileImpl<llvm::MemoryBuffer>(int, llvm::Twine const&, unsigned long long, unsigned long long, long long, bool, bool, llvm::Optional<llvm::Align>) (.cold.1) + 0
8  clang-tblgen             0x000000010464a548 llvm::WritableMemoryBuffer::getNewUninitMemBuffer(unsigned long, llvm::Twine const&, llvm::Optional<llvm::Align>) + 496
9  clang-tblgen             0x000000010464a7a0 llvm::ErrorOr<std::__1::unique_ptr<llvm::MemoryBuffer, std::__1::default_delete<llvm::MemoryBuffer>>> getOpenFileImpl<llvm::MemoryBuffer>(int, llvm::Twine const&, unsigned long long, unsigned long long, long long, bool, bool, llvm::Optional<llvm::Align>) + 528
10 clang-tblgen             0x000000010464a254 llvm::ErrorOr<std::__1::unique_ptr<llvm::MemoryBuffer, std::__1::default_delete<llvm::MemoryBuffer>>> getFileAux<llvm::MemoryBuffer>(llvm::Twine const&, unsigned long long, unsigned long long, bool, bool, bool, llvm::Optional<llvm::Align>) + 176
11 clang-tblgen             0x000000010466fbbc llvm::TableGenMain(char const*, bool (*)(llvm::raw_ostream&, llvm::RecordKeeper&)) + 1428
12 clang-tblgen             0x0000000104626114 main + 156
13 dyld                     0x0000000187453e50 start + 2544