https://github.com/llvm/llvm-project/pull/173869 accidentally dropped
rtti and eh flags from `llvm-config --cxxflags`. Then
https://github.com/llvm/llvm-project/pull/174084 restored the rtti
flags. The eh flags were not included with the rationale that they are
not ABI relevant.
This PR restores the eh flags as well. While they are not strictly
necessary, I believe that code that directly interfaces with LLVM almost
certainly does not want to build with exceptions if LLVM is not built
with exceptions. Building in the peculiar `-fexceptions -fno-rtti`
configuration is rarely useful and likely not intended.
On MacOS, this is also relevant because it's not possible to use C++17
headers without `-fno-exceptions` when using older deployment targets.
In that configuration, `-fno-exceptions` is required to interact with
LLVM.
9bbea753a4