Fix "process load" on new android targets

Summary:
On older android targets, we needed a dlopen rename workaround to get
"process load" working. Since API 26 this is not required as the targets
have a proper libdl so with the function names one would expect.

To make this work I've had to remove the const qualifier from the
GetLibdlFunctionDeclarations function (as now the declarations can
depend on the connected target). Since I was already modifying the
prototype (and the lower levels were already converted to StringRef) I
took the oportunity to convert this function as well.

llvm-svn: 307160