[clang] [clang-tools-extra] [flang] Fix OOM in FormatDiagnostic (2nd attempt) (PR #108866)

[Ivan Kosarev via cfe-commits]

kosarev wrote:

@igelbox This seems to cause `Clang-Unit :: Rename/./ClangRenameTests/*` failures on my machine. Still reproduces on the current top of branch, dbe47c2a06e0928edde802d062ecf1a0ce45fbb9. Anyone else can see the failures?

The `Clang-Unit :: AST/Interp/./InterpTests/*` failures are from before.

```
$ ninja check-clang-unit
...
Failed Tests (38):
  Clang-Unit :: AST/Interp/./InterpTests/1/5
  Clang-Unit :: AST/Interp/./InterpTests/2/5
  Clang-Unit :: AST/Interp/./InterpTests/3/5
  Clang-Unit :: AST/Interp/./InterpTests/4/5
  Clang-Unit :: AST/Interp/./InterpTests/Descriptor/Primitives
  Clang-Unit :: Rename/./ClangRenameTests/0/33
  Clang-Unit :: Rename/./ClangRenameTests/1/33
  Clang-Unit :: Rename/./ClangRenameTests/10/33
  Clang-Unit :: Rename/./ClangRenameTests/11/33
  Clang-Unit :: Rename/./ClangRenameTests/12/33
  Clang-Unit :: Rename/./ClangRenameTests/13/33
  Clang-Unit :: Rename/./ClangRenameTests/14/33
  Clang-Unit :: Rename/./ClangRenameTests/15/33
  Clang-Unit :: Rename/./ClangRenameTests/16/33
  Clang-Unit :: Rename/./ClangRenameTests/17/33
  Clang-Unit :: Rename/./ClangRenameTests/18/33
  Clang-Unit :: Rename/./ClangRenameTests/19/33
  Clang-Unit :: Rename/./ClangRenameTests/2/33
  Clang-Unit :: Rename/./ClangRenameTests/20/33
  Clang-Unit :: Rename/./ClangRenameTests/21/33
  Clang-Unit :: Rename/./ClangRenameTests/22/33
  Clang-Unit :: Rename/./ClangRenameTests/23/33
  Clang-Unit :: Rename/./ClangRenameTests/24/33
  Clang-Unit :: Rename/./ClangRenameTests/25/33
  Clang-Unit :: Rename/./ClangRenameTests/26/33
  Clang-Unit :: Rename/./ClangRenameTests/27/33
  Clang-Unit :: Rename/./ClangRenameTests/28/33
  Clang-Unit :: Rename/./ClangRenameTests/29/33
  Clang-Unit :: Rename/./ClangRenameTests/3/33
  Clang-Unit :: Rename/./ClangRenameTests/30/33
  Clang-Unit :: Rename/./ClangRenameTests/31/33
  Clang-Unit :: Rename/./ClangRenameTests/32/33
  Clang-Unit :: Rename/./ClangRenameTests/4/33
  Clang-Unit :: Rename/./ClangRenameTests/5/33
  Clang-Unit :: Rename/./ClangRenameTests/6/33
  Clang-Unit :: Rename/./ClangRenameTests/7/33
  Clang-Unit :: Rename/./ClangRenameTests/8/33
  Clang-Unit :: Rename/./ClangRenameTests/9/33


Testing Time: 5.04s

Total Discovered Tests: 18068
  Skipped:     4 (0.02%)
  Passed : 18026 (99.77%)
  Failed :    38 (0.21%)
```

```
cmake \
  -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
  -DCMAKE_C_COMPILER=clang \
  -DCMAKE_CXX_COMPILER=clang++ \
  -DLLVM_ENABLE_WERROR=OFF \
  -DLLVM_USE_LINKER=gold \
  -DBUILD_SHARED_LIBS=ON \
  -DCMAKE_BUILD_TYPE=RELEASE \
  -DCMAKE_INSTALL_PREFIX=.. \
  -DLLVM_ENABLE_ASSERTIONS=ON \
  -DLLVM_ENABLE_PROJECTS=clang \
  -GNinja \
```

https://github.com/llvm/llvm-project/pull/108866