Set the EmulatedTLS option based on `Triple::hasDefaultEmulatedTLS()`
if the user didn't specify it; set `ExplicitEmulatedTLS` to true
in `llvm::TargetOptions` and set `EmulatedTLS` to Clang's
opinion of what the default or preference is.
This avoids any risk of deviance between the two.
This affects one check of `getCodeGenOpts().EmulatedTLS` in
`shouldAssumeDSOLocal` in CodeGenModule, but as that check only
is done for `TT.isWindowsGNUEnvironment()`, and
`hasDefaultEmulatedTLS()` returns false for such environments
it doesn't make any current testable difference - thus NFC.
Some mingw distributions carry a downstream patch, that enables
emulated TLS by default for mingw targets in `hasDefaultEmulatedTLS()`
- and for such cases, this patch does make a difference and fixes the
detection of emulated TLS, if it is implicitly enabled.
Differential Revision: https://reviews.llvm.org/D132916
fbfe1db4a9