[llvm] [C-API] Allow SectionMemoryManager's ReserveAlloc flag to be enabled (PR #174307)

[Thomas Munro via llvm-commits]

macdice wrote:

Presumably needs <stdbool.h>, not picked up by regular CI or build.  Or is there some reason to prefer another type eg int here?

https://github.com/llvm/llvm-project/pull/174307