[BRIGFE] Enable whole program optimizations

[BRIGFE] Enable whole program optimizations

HSA assumes all program scope HSAIL symbols can be queried from
the host runtime API, thus cannot be removed by the IPA.

Getting some inlining happening in the finalized binary required:
* explicitly marking the 'prog' scope functions and the launcher
function "externally_visible" to avoid the inliner removing it
* also the host_def ptr is set to externally visible, otherwise
IPA assumes it's never set
* adding the 'inline' keyword to functions to enable inlining,
otherwise GCC defaults to replaceable functions (one can link
over the previous one) which cannot be inlined
* replacing all calls to declarations with calls to definitions to
enable the inliner to find the definition
* to fix missing hidden argument types in the generated functions.
These were ignored silently until GCC started to be able to
inline calls to such functions.
* do not gimplify before fixing the call targets. Otherwise the
calls get detached and the definitions are not found. The reason
why this happens is not clear, but gimplifying only after call
target decl->def conversion fixes this.

From-SVN: r259943