[analyzer] Fix an assertation failure for invalid sourcelocation, add a new debug checker

For a rather short code snippet, if debug.ReportStmts (added in this patch) was
enabled, a bug reporter visitor crashed:

struct h {
  operator int();
};

int k() {
  return h();
}

Ultimately, this originated from PathDiagnosticLocation::createMemberLoc, as it
didn't handle the case where it's MemberExpr typed parameter returned and
invalid SourceLocation for MemberExpr::getMemberLoc. The solution was to find
any related valid SourceLocaion, and Stmt::getBeginLoc happens to be just that.

Differential Revision: https://reviews.llvm.org/D58777

llvm-svn: 356161
diff --git a/clang/docs/analyzer/developer-docs/DebugChecks.rst b/clang/docs/analyzer/developer-docs/DebugChecks.rst
index bb2f37f..56ce015 100644
--- a/clang/docs/analyzer/developer-docs/DebugChecks.rst
+++ b/clang/docs/analyzer/developer-docs/DebugChecks.rst
@@ -285,3 +285,10 @@
 statistics within the analyzer engine. Note the Stats checker (which produces at
 least one bug report per function) may actually change the values reported by
 -analyzer-stats.
+
+Output testing checkers
+=======================
+
+- debug.ReportStmts reports a warning at **every** statement, making it a very
+  useful tool for testing thoroughly bug report construction and output
+  emission.