SwiftCompilerSources: rework bridging

Introduce two modes of bridging:
* inline mode: this is basically how it worked so far. Using full C++ interop which allows bridging functions to be inlined.
* pure mode: bridging functions are not inlined but compiled in a cpp file. This allows to reduce the C++ interop requirements to a minimum. No std/llvm/swift headers are imported.

This change requires a major refactoring of bridging sources. The implementation of bridging functions go to two separate files: SILBridgingImpl.h and OptimizerBridgingImpl.h.
Depending on the mode, those files are either included in the corresponding header files (inline mode), or included in the c++ file (pure mode).

The mode can be selected with the BRIDGING_MODE cmake variable. By default it is set to the inline mode (= existing behavior). The pure mode is only selected in certain configurations to work around C++ interop issues:
* In debug builds, to workaround a problem with LLDB's `po` command (rdar://115770255).
* On windows to workaround a build problem.

SwiftCompilerSources/Sources/SIL/BasicBlock.swift

@@ -21,8 +21,7 @@ final public class BasicBlock : CustomStringConvertible, HasShortDescription, Eq
   public var parentFunction: Function { bridged.getFunction().function }
 
   public var description: String {
-    let stdString = bridged.getDebugDescription()
-    return String(_cxxString: stdString)
+    return String(taking: bridged.getDebugDescription())
   }
   public var shortDescription: String { name }