[Bridging PCH] Terminology clarification as suggested in review.

Author: graydon

docs/Lexicon.rst

@@ -231,6 +231,17 @@ source code, tests, and commit messages. See also the `LLVM lexicon`_.
     library on the system when the library on the system cannot be modified.
     Apple has a number of overlays for its own SDKs in stdlib/public/SDK/.
 
+  PCH
+    Precompiled header, a type of file ending in .pch. A precompiled header is
+    like a precompiled module, in the sense that it's the same file format and
+    is just a cache file produced by clang and read by ``clang::ASTReader``. The
+    difference is that PCH files are not "modular": they do not correspond to a
+    named module, and cannot be read in any order or imported by module-name;
+    rather they must be the first file parsed by the compiler. PCHs are used
+    only to accelerate the process of reading C/C++/Objective-C headers, such as
+    the bridging headers read in by the ``-import-objc-header`` command-line
+    flag to swiftc.
+
   PR
     1. "Problem Report": An issue reported in `LLVM's bug tracker`__.
        See also `SR`.

include/swift/AST/DiagnosticsClangImporter.def

@@ -54,7 +54,7 @@ WARNING(could_not_rewrite_bridging_header,none,
   "failed to serialize bridging header; "
   "target may not be debuggable outside of its original project", ())
 ERROR(bridging_header_pch_error,Fatal,
-   "failed to emit PCH file '%0' for bridging header '%1'",
+   "failed to emit precompiled header '%0' for bridging header '%1'",
    (StringRef, StringRef))
 
 WARNING(invalid_swift_name_method,none,