EE/Interpreter/ExternalFunctions.cpp: Prune "C" linkage to suppress warnings with...

EE/Interpreter/ExternalFunctions.cpp: Prune "C" linkage to suppress warnings with -Wreturn-type (and MSC's w4190).

In historical reason, Interpreter's external entries had prefix "lle_X_" as C linkage, even for well-known entries in EE/Interpreter.
Now, at least on ToT, they are resolved via FuncNames[] mapper.
We will not need their symbols are expected to be exported any more.

Clang r150128 has introduced the warning <"%0 has C-linkage specified, but returns user-defined type %1 which is incompatible with C">.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151312 91177308-0d34-0410-b5e6-96231b3b80d8