[docs] Add a little section about Disabling PCH Verification.

Author: gottesmm

docs/DebuggingTheCompiler.md

@@ -36,6 +36,7 @@ benefit of all Swift developers.
         - [Bisecting on SIL optimizer pass counts to identify optimizer bugs](#bisecting-on-sil-optimizer-pass-counts-to-identify-optimizer-bugs)
         - [Using git-bisect in the presence of branch forwarding/feature branches](#using-git-bisect-in-the-presence-of-branch-forwardingfeature-branches)
         - [Reducing SIL test cases using bug_reducer](#reducing-sil-test-cases-using-bug_reducer)
+    - [Disabling PCH Verification](#disabling-pch-verification)
 - [Debugging the Compiler Build](#debugging-the-compiler-build)
     - [Build Dry Run](#build-dry-run)
 - [Debugging the Compiler Driver](#debugging-the-compiler-driver-build)
@@ -832,6 +833,19 @@ each time you change shas. To do this you can pass `--match-timestamp`
 to automatically checkout match the timestamp of the `apple/swift` repo
 across the other repos.
 
+## Disabling PCH Verification
+
+Sometimes one needs to try to compile against PCH modules where the PCH version
+verification checking is too strict. To work around this, one can disable the
+checking by passing in to swift:
+
+```sh
+-Xcc -Xclang -Xcc -fno-validate-pch
+```
+
+NOTE: If there are actual differences in between the on disk PCH format and the
+format expected by the compiler crashes and undefined behavior may result.
+
 # Debugging the Compiler Build
 
 ## Build Dry Run