[llvm-jitlink] Use '@' rather than ':' for separator in -sectcreate.

This should avoid the issue with Windows paths that have caused failures on
some builders.

Author: lhames

llvm/test/ExecutionEngine/JITLink/Generic/sectcreate.test

@@ -1,6 +1,6 @@
 # RUN: llc -filetype=obj -o %t.o %S/Inputs/main-ret-0.ll
 # RUN: llvm-jitlink -noexec \
-# RUN:   -sectcreate __data,%S/Inputs/sectcreate-data.txt:foo=0 \
+# RUN:   -sectcreate __data,%S/Inputs/sectcreate-data.txt@foo=0 \
 # RUN:   %t.o
 #
 # Use -sectcreate to create a section from a data file.

llvm/tools/llvm-jitlink/llvm-jitlink.cpp

@@ -165,7 +165,7 @@ static cl::list<std::string>
 
 static cl::list<std::string>
     SectCreate("sectcreate",
-               cl::desc("given <sectname>,<filename>[:<sym>=<offset>,...]  "
+               cl::desc("given <sectname>,<filename>[@<sym>=<offset>,...]  "
                         "add the content of <filename> to <sectname>"),
                cl::cat(JITLinkCategory));
 
@@ -1683,7 +1683,7 @@ static Error addSectCreates(Session &S,
 
     StringRef SCArg(*SCItr);
 
-    auto [SectAndFileName, ExtraSymbolsString] = SCArg.split(':');
+    auto [SectAndFileName, ExtraSymbolsString] = SCArg.split('@');
     auto [SectName, FileName] = SectAndFileName.rsplit(',');
     if (SectName.empty())
       return make_error<StringError>("In -sectcreate=" + SCArg +