[ORC] Turn on symbol-flags overrides for LLJIT on Windows by default.

[ORC] Turn on symbol-flags overrides for LLJIT on Windows by default.

libObject does not apply the Exported flag to symbols in COFF object files,
which can lead to assertions when the symbol flags initially derived from
IR added to the JIT clash with the flags seen by the JIT linker. Both
RTDyldObjectLinkingLayer and ObjectLinkingLayer have a workaround for this:
they can be told to override the flags seen by the linker with the flags
attached to the materialization responsibility object that was passed down
to the linker. This patch modifies LLJIT's setup code to enable this override
by default on platforms where COFF is the default object format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367712 91177308-0d34-0410-b5e6-96231b3b80d8