subsume the "startsWithGreater" logic into skipUntilGreaterInTypeList, and

make sure skipUntilGreaterInTypeList return a valid location even in the case
of parse errors.  This ensures that we form a valid source range.  Also,
improve parseExprIdentifier() to handle the case when skipUntilGreaterInTypeList
returns an empty archetype argument list.  This fixes a couple of compiler
crashers.

validation-test/SIL/crashers_fixed/019-swift-parser-parseexprpostfix.sil

@@ -0,0 +1,2 @@
+// RUN: not --crash %target-sil-opt %s
+C<@convention()>{