[llvm] Fix small typos in Programmer's Manual and Chrono.h.

[noajshu]

Summary

The Programmer's Manual guidance on the StringRef class (https://llvm.org/docs/ProgrammersManual.html#the-stringref-class) refers to iterator StringRef::find(StringRef Key) which does not exist. Based on context this is surely a small typo meant to be iterator StringMap::find(StringRef Key).

This also corrects some small typos in the comments of llvm/include/llvm/Support/Chrono.h

[noajshu]

Also fix some typos in Chrono.h

[phosek]

LGTM

llvm/docs/ProgrammersManual.rst

 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The ``StringRef`` data type represents a reference to a constant string (a
 character array and a length) and supports the common operations available on
 ``std::string``, but does not require heap allocation.
 
 It can be implicitly constructed using a C style null-terminated string, an
 ``std::string``, or explicitly with a character pointer and length.  For
-example, the ``StringRef`` find function is declared as:
+example, the ``StringMap`` find function is declared as:
 
 .. code-block:: c++
 
   iterator find(StringRef Key);
 
 and clients can call it using any one of:
 
 .. code-block:: c++

llvm/include/llvm/Support/Chrono.h

 
 namespace llvm {
 
 class raw_ostream;
 
 namespace sys {
 
 /// A time point on the system clock. This is provided for two reasons:
-/// - to insulate us agains subtle differences in behavoir to differences in
-///   system clock precision (which is implementation-defined and differs between
-///   platforms).
+/// - to insulate us against subtle differences in behavior to differences in
+///   system clock precision (which is implementation-defined and differs
+///   between platforms).
 /// - to shorten the type name
-/// The default precision is nanoseconds. If need a specific precision specify
-/// it explicitly. If unsure, use the default. If you need a time point on a
-/// clock other than the system_clock, use std::chrono directly.
+/// The default precision is nanoseconds. If you need a specific precision
+/// specify it explicitly. If unsure, use the default. If you need a time point
+/// on a clock other than the system_clock, use std::chrono directly.
 template <typename D = std::chrono::nanoseconds>
 using TimePoint = std::chrono::time_point<std::chrono::system_clock, D>;
 
 /// Convert a TimePoint to std::time_t
 inline std::time_t toTimeT(TimePoint<> TP) {
   using namespace std::chrono;
   return system_clock::to_time_t(
       time_point_cast<system_clock::time_point::duration>(TP));