[Support] Use malloc instead of non-throwing new (#92157)

When allocating a memory buffer, we use a non-throwing new so that we
can explicitly handle memory buffers that are too large to fit into
memory. However, when exceptions are disabled, LLVM installs a custom
new handler

(https://github.com/llvm/llvm-project/blob/90109d444839683b09f0aafdc50b749cb4b3203b/llvm/lib/Support/InitLLVM.cpp#L61)
that explicitly crashes when we run out of memory

(https://github.com/llvm/llvm-project/blob/de14b749fee41d4ded711e771e43043ae3100cb3/llvm/lib/Support/ErrorHandling.cpp#L188)
and that means this particular out-of-memory situation cannot be
gracefully handled.

This was discovered while working on #embed
(https://github.com/llvm/llvm-project/pull/68620) on Windows and
resulted in a crash rather than the preprocessor issuing a diagnostic as
expected.

This patch switches away from the non-throwing new to a call to malloc
(and free), which will return a null pointer without calling a custom
new han...