[MS] Push fewer DeclContexts for delayed template parsing

Only push the outermost record as a DeclContext when parsing a function
body. See the comments in Sema::getContainingDC about the way the parser
pushes contexts. This is intended to match the behavior the parser
normally displays where it parses all method bodies from all nested
classes at the end of the outermost class, when all nested classes are
complete.

Fixes PR38460.

llvm-svn: 347627